Cage vs Ring is a factor. Both should have been a division lower than they were too when they moved to the UFC IMO.
They also had a ridiculous amount of wear and tear for guys in their early thirties. First, they were both basically kill or be killed type of fighters, which was encouraged in Japan. They'd both been badly KOd prior to coming to the USA.
CroCop had over 50 K1/MMA fights, which is crazy. Basically 5 fights/year for 10 years straight. Wanderei was the same with like 40 MMA fights and training hard at Chutebox.
And they did can crush a bit in Pride, but it wasn't like they had 15 easy fights at local shows before they hit the big time. Compared to guys that got into the UFC relatively early in their careers they had an absurd number of fights. Chris Weidman, Leben, Uriah Hall, Dominick Reyes: WAY less fights by age 30.
By like age 31 CroCop had fought Barnett x 2, Hunt, Wanderlei x 2, Coleman, Fedor, Alex E, Randelman x 2, Big Nog, Igor, Fujita, Herring. And in K1: Hunt, Bojansky, Aerts, Hoost x 3, Le Banner. And about 25 other dudes.
CroCop had 14 MMA fights in 2004 and 2005!! Imagine Tom Aspinall doing that, or Jiri P. It's crazy, even with PEDs. Colby Covington has been in the UFC for 10 years and has only 15 fights total, by comparison.
Wanderlei had 10 MMA fights in 2000 and 2001, against Tito, Guy Mezger, Yvel, Hendo, Sakuraba x 2.