Not singling out anyone here, particularly since many aren't doing it, but our fans, like many others, overestimate our players' NFL prospects frequently. The NFL is ruthless and places a great deal of emphasis on measurables, only some of which can be improved at all, and even then only in part, with everyone else working to improve them as well. People underestimate how much talent these other programs are churning out. It's a self-perpetuating cycle, where these other programs are loaded with recruits with NFL genes, they get drafted and have some pan out in the NFL, then tout that they "get players to the NFL" in order to help them recruit even more guys with NFL genes. There are a few exceptions out there in which programs have been able to get lower-rated recruits to the NFL disproportionately (like Utah with DBs), but that pretty much is the game.
The vast majority of the time, WSU instead is going to be landing recruits who are not NFL material but helping them maximize their talent and, as a benefit of having them often not NFL-level talents, stick around and produce at a high level as upperclassmen. It sucks when these guys leave school early to play in the NFL when they don't make it. That's what Leach didn't like with Williams' decision, although in fairness to Williams, there isn't much more he could have shown as a senior to change his status, either.
It's pretty simple with Williams: he just isn't quite good enough. If the threshold is a "90" on a variety of metrics to make a 53-man roster as a RB, he's somewhere in the 80s on many of them, with some aspects more in the high 70s. Very good, and right on the cusp -- not to mention being a great kid who worked really hard, and who I don't begrudge for trying to provide for his family -- but just not enough to be a RB in the NFL. There are just too many guys like him pumped out every year. It's tempting to watch apparent stiffs who seem to stick around in the NFL despite not being that good, like some of the recycled backup QBs, etc., but in addition to that sometimes just being a product of circumstances in some cases, those guys usually are better than it seems. We're just used to analyzing them relative to the complete freak athletes in the NFL rather than taking good college players' performances against other college kids and making (flawed) projections about what they can do in the NFL.