Because, only idiots actually think that. He was drafted by a team with an already stacked D-line and for the few (one IIRC) available spots got beat out by an undrafted FA. His measurables weren't up to par with what people expect. He wasn't fast enough for his size (too slow) and he wasn't strong or large enough for his lack of speed (too small). He was a 'tweener', or 'inbetweener'. He also seemed to have the attitude that being openly gay is what was holding him back, but after being in the NFL for awhile (on a practice squad), it looks like it humbled him enough to realize he wasn't the 2nd or 3rd round pick he believed he should have been. He was, at best, a situational pass rusher.
If anything, I believe the Rams picked him to make a point in history and a statement about their organization more than they needed him. A few teams were reportedly interested in him, but not willing to use a draft pick and attempt to pick him up in free agency. Since the Rams took his rights and by the time he was cut (last cut before the final 53 roster), the teams interested already had other prospects they liked. Dallas, seeing a potential need in their awful defense, picked him up as a practice player in case they needed him. It didn't work out there, as they ended up not needing him, and eventually needed LBs instead and had to cut him.