Simple.
Fedor wasn't as good as his legion of devoted followers made him out to be.
This became even more frustrating when Pride dissolved and anyone worth their salt took their chance in the UFC, except for Fedor... who resisted in order to cherry pick fights in his own org.
Finally when Strikeforce came around and pitted him against relevant current heavyweights, he lost, and lost bad, at the age of 33. To other fighters around the same age.
He was absolutely the greatest in 2002 to about 2008, but at about that time, I feel there were a list of more well rounded HWs that could beat him. There's no way a prime Fedor dominates the current HW landscape like he did back then.
He's still the HW GOAT based on his accomplishments.