Why aren't there any middle relievers in the Hall of Fame?

I've been thinking about middle relievers a lot lately. The red-headed stepchildren of Major League Baseball, relievers are often little more than an afterthought when it comes to discussions of the game's greats. The general sentiment is, if he's such a great pitcher, then why isn't he a starter, or at least a closer? To examine the game's best middle men and to see if perhaps any of them might merit, at the very least, some HOF consideration, I turned to BBref's Play Index.

So, what makes a successful relief pitcher? I decided, rather arbitrarily (yet conservatively) that a relief pitcher can be said to have had a successful season if he were to relieve at least 50 games, and record an ERA under 4. Who do you think has the most seasons in his career fulfilling these criteria?

                  From  To   Ages Seasons Link to Individual Seasons
+-----------------+----+----+-----+-------+------------------------------+
Jesse Orosco      1982 2002 25-45      15 Ind. Seasons                  
 Lee Smith         1982 1996 24-38      14 Ind. Seasons                  
 Trevor Hoffman    1993 2007 25-39      13 Ind. Seasons                  
 Hoyt Wilhelm      1952 1970 29-47      13 Ind. Seasons                  
 John Franco       1984 2000 23-39      12 Ind. Seasons                  
 Kent Tekulve      1976 1988 29-41      12 Ind. Seasons                  
 Mariano Rivera    1996 2007 26-37      11 Ind. Seasons                  
 Dan Plesac        1986 2003 24-41      10 Ind. Seasons                  
 Gary Lavelle      1975 1985 26-36      10 Ind. Seasons                  
 Gene Garber       1974 1986 26-38      10 Ind. Seasons                  
 Sparky Lyle       1969 1979 24-34      10 Ind. Seasons                  
 Rollie Fingers    1969 1982 22-35      10 Ind. Seasons                  
 Ron Perranoski    1961 1970 25-34      10 Ind. Seasons                  
 Roy Face          1956 1967 28-39      10 Ind. Seasons                  
 Billy Wagner      1997 2007 25-35       9 Ind. Seasons                  
 Troy Percival     1995 2004 25-34       9 Ind. Seasons                  
 Bob Wickman       1994 2007 25-38       9 Ind. Seasons                  
 Mike Timlin       1991 2007 25-41       9 Ind. Seasons                  
 Mike Jackson      1988 2002 23-37       9 Ind. Seasons                  
 Paul Assenmacher  1986 1998 25-37       9 Ind. Seasons                  
 Craig Lefferts    1983 1991 25-33       9 Ind. Seasons                  
 Jeff Reardon      1980 1992 24-36       9 Ind. Seasons                  
 Clay Carroll      1966 1977 25-36       9 Ind. Seasons  

An interesting list, but it is polluted with closers. Let's add the criteria that the pitcher may have had no more than 15 saves in the season - taking over as closer for a few weeks is probably not enough to bump him into closer status, but much more than that and he's a closer.

Voila!

                   From  To   Ages Seasons Link to Individual Seasons
+-----------------+----+----+-----+-------+------------------------------+
Jesse Orosco      1982 2002 25-45      11 Ind. Seasons                  
 Paul Assenmacher  1986 1998 25-37       9 Ind. Seasons                  
 Hoyt Wilhelm      1952 1970 29-47       9 Ind. Seasons                  
 Steve Kline       1998 2006 25-33       8 Ind. Seasons                  
 Steve Reed        1994 2004 29-39       8 Ind. Seasons                  
 Mike Jackson      1988 2002 23-37       8 Ind. Seasons                  
 Mike Stanton      1991 2006 24-39       7 Ind. Seasons                  
 Rick Honeycutt    1988 1996 34-42       7 Ind. Seasons                  
 Dan Plesac        1986 2003 24-41       7 Ind. Seasons                  
 Kent Tekulve      1976 1988 29-41       7 Ind. Seasons                  
 Gary Lavelle      1975 1985 26-36       7 Ind. Seasons                  
 Darold Knowles    1966 1978 24-36       7 Ind. Seasons  

What do you notice about this list?  I see a bunch of LOOGYs.  Let's pare it down to strictly right-handers:

                   From  To   Ages Seasons Link to Individual Seasons
+-----------------+----+----+-----+-------+------------------------------+
Hoyt Wilhelm      1952 1970 29-47       9 Ind. Seasons                  
 Steve Reed        1994 2004 29-39       8 Ind. Seasons                  
 Mike Jackson      1988 2002 23-37       8 Ind. Seasons                  
 Kent Tekulve      1976 1988 29-41       7 Ind. Seasons                  
 Dave Weathers     2000 2006 30-36       6 Ind. Seasons                  
 Julian Tavarez    1995 2005 22-32       6 Ind. Seasons                  
 Jeff Nelson       1992 2003 25-36       6 Ind. Seasons                  
 Mike Timlin       1991 2007 25-41       6 Ind. Seasons                  
 Mark Eichhorn     1986 1993 25-32       6 Ind. Seasons                  
 Larry Andersen    1984 1993 31-40       6 Ind. Seasons                  
 Gene Garber       1974 1985 26-37       6 Ind. Seasons                  
 Clay Carroll      1966 1977 25-36       6 Ind. Seasons                  
 Bob Locker        1965 1972 27-34       6 Ind. Seasons                  
 Don McMahon       1959 1967 29-37       6 Ind. Seasons                  
 LaTroy Hawkins    2000 2007 27-34       5 Ind. Seasons                  
 Jim Mecir         1998 2005 28-35       5 Ind. Seasons                  
 Scott Sullivan    1997 2003 26-32       5 Ind. Seasons                  
 Paul Quantrill    1997 2003 28-34       5 Ind. Seasons                  
 Dave Veres        1995 2002 28-35       5 Ind. Seasons                  
 Jeff Parrett      1988 1996 26-34       5 Ind. Seasons                  
 Gene Nelson       1986 1993 25-32       5 Ind. Seasons                  
 Greg Harris       1985 1993 29-37       5 Ind. Seasons                  
 Ron Reed          1976 1984 33-41       5 Ind. Seasons                  
 Randy Moffitt     1973 1978 24-29       5 Ind. Seasons                  
 Pedro Borbon      1972 1976 25-29       5 Ind. Seasons                  
+-----------------+----+----+-----+-------+------------------------------+
                   From  To   Ages Seasons Link to Individual Seasons
+-----------------+----+----+-----+-------+------------------------------+
Eddie Fisher      1964 1971 27-34       5 Ind. Seasons                  
 Lindy McDaniel    1959 1969 23-33       5 Ind. Seasons                  
 Don Elston        1958 1963 29-34       5 Ind. Seasons                  
 Turk Farrell      1957 1968 23-34       5 Ind. Seasons      

Hoys Wilhelm is a Hall of Famer, but because of his exploits as a closer - he just pitched so damn long, he was just able to have so many other seasons in which he didn't save a lot of games. So there you have it - there are no HOF middle relievers, because none of them were ever able to string together as many as ten "good" seasons as middle relievers.