Because 'Fel' has been ****ed since Ren and the vast majority of content has been designed with Tram in mind.
Why? Because some people didn't like UO and were determined to change it, despite the the hundreds of thousands of people who enjoyed it. Rather than playing something else, something that suited their gamestyle, they pushed for UO to be changed into something that not only did not suit the game, it caused (and continues to cause) major problems, and endless debate... EA paniked (wouldnt be the only time, UO2, UXO anyone?) in the face of competion (EQ). The rest, as they say, is history.
Garriot left, Britannia was left kingless. The playerbase was forever split in two, community interaction was left as a shadow of its former self. Tram was introduced, a dull, boring copy of the world that already existed (Ilsh was vast improvement in this department however). Players were now able to play the game in 'easy mode,' without real fear of loss, this also made it easier for scripters to ply their trade as player justice was removed from the new lands.
Before Ren subscribers were on a constant, steady increase. It was only after the introduction of Tram (to be fair it was fair while after and probably more closly tied to AoS) that subscribers would fall for the first time.
I think what he means is that UO is known for its PvP, rather than anything else. The level of skill required to PvP well (especially on a mage) is far greater than any other MMO that I am aware of.
Taken from UO's wikipedia page...