First post got it right.
Cro Cop was already contemplating retirement around the time of the Open Weight GP, Wanderlei had just got done getting VICIOUSLY, CAREER-THREATENINGLY (the are you dead? type) ktfo by a highlight reel Cro Cop LHK and then again by a clean Hendo H-Bomb, Shogun suffered his first ACL rupture during the layoff during the UFC-Pride merger, Gomi was already on a downward spiral in his career coming into the UFC, etc.
All that combined with the fact that you take this declining fighters and put them in a COMPLETELY new environment in the form of the UFC octagon (far different from a ring in surface area as well as the ability too cut corners/cut your opponent off) and subject them to rule changes which for some (Shogun and Wand...that CB style) forced them to almost COMPLETELY have to reinvent themselves as fighters after having some of their most trademarked weapons be made illegal.
That's just the truth. You're gonna have UFC fanboys spout their nonsense because the buyout and migration happened the way it did and it wasn't the UFC that was bought out but objectively it was towards the end of an era.
Also how many "UFC guys" really did well over in Pride? Not many. Prime Chuck for instance looked VERY human against Overeem (lucky for Chuck, Overeem did his usual procedure of gassing himself into a knockout loss) and of course got beat badly by Rampage. Guy Mezger? Also had a horrible record in Pride. Vitor got bounced out of the opening round of 05 MW/LHW GP too iirc. Got submitted by Overeem in the first round.
I mean most UFC fighters who came over to Pride did just about equally as bad. Even worse because a lot of those guys were young and/or not as over the hill as they were to become.