It's a MUCH longer learning curve, and in a salary-capped league, teams have to have quality guys on ELCs. Other issue is, post-lockout, the crackdown on interference in the neutral zone means forwards are bearing down on D-men much quicker on dump-ins, and a heavy forecheck can cause serious problems.
A plodding, heavy D *could* survive pre-lockout because a winger would be able to slow a charging forward down before he got to the corner, and the two-line pass meant he didn't have to cover quite as much space in the neutral zone. A smaller, quicker, offensive-minded D could survive because he could make a play before the forward came flying in.
Good all-around D are pretty rare, but because of the increased speed of the game, a mid-level guy gets exposed more, and teams tend to lock up their defensemen early and not let them move.
Our local ECHL team - a Chicago affiliate - almost always has 2 defensemen assigned from the AHL/NHL team on the roster (and sometimes three), but often only one forward (if that). Forwards don't have much of a learning curve. Defensemen have a steep one, so teams need to be more patient with them ... but the ELC setup doesn't really allow teams to be more patient as they become RFAs within 3 years, and UFAs if they stay in the minors for a set period of time.
Not surprisingly, two of the changes Bobby Orr suggested in his book were to allow forwards to interfere a little bit and to reinstate the 2-line pass, both of which would slow the game down a bit and give defensemen more of a chance.