They made a big deal of him making his debut at Madison Square Garden, saying it was the one place left he wanted to wrestle in. Then he just became typical cruiserweight fodder afterwards. Anyway, the fingerprints he left on the wrestling world is undeniable.
It's not just that, but when you add in that he was a shell of his WCW time after his arm surgery and more or less 4-5 years off, and him being hyped like he was another Rey Jr., it couldn't have worked out. Ultimo wasn't really much of a high flying cruiser outside of the Asai Moonsault. But he was hyped as "Rey Jr, but Japanese!" Which obviously wasn't the case at all. Plus the WWE midcard style was not anything like the WCW midcard style, so even when he had matches against WCW guys, it was super dumbed down and neutered. I don't think his style ever was going to translate to WWE like Rey/Eddie/Tajiri's styles did. Add in a lack of similar opponents (even though most of the best WCW cruisers from 1996-2000 were in WWE) and you had something that was doomed to fail. I was so hyped for his comeback, too.