Probably because not many of those guys are good enough athletes.
The more complex NFL schemes become, the more important it is to have smart - reasonably smart - players on the field. Back in the 60s and earlier, a lot of those guys didn't feel comfortable signing their names unless they had a crayon in their hands.
For example, the wonderlic test...I read somewhere that the average NFL player scores higher on the wonderlic than the average dude in the general population. The modern NFL is, as a whole, filled with relatively bright guys.