Constitution's problem is that with how much random bullshit can catch you out of the blue (traps, lots of enemies, crawlers, other stealthy enemies) most people would rather savescum and try again to strike with overwhelming force. The addition of veteran feats in Expedition probably caused people to dump it even more because of feats that require more than 10 points in a base stat, so points that could've been used to round up the character rather than squeeze +9% damage are now used for +10 AP on Sprint. "Can openers" like several kinds of psionic spells make it even less enticing to pick up, the best defense is not getting hit at all and you don't get hit at all by killing the enemy as soon as possible and abusing corners that break line of sight, something tanky Constitution builds generally can't rely on as much compared to more lethal and more mobile builds.
