Taz's whole thing was unsafe suplexes and being super stiff to make him seem like a bad ass shooter. Literally his gimmick. Along with long promos where he constantly had to be censored. All that is fine in grimy ass ECW with dudes like Sabu and Tommy Dreamer and Sandman, but none of that was ever going to work in WWE. This was a problem most every ECW guy had, in addition to the size differences that always stood out dramatically when in a WWE ring against those guys. On top of all that, 4 guys from WCW jumped right around the same time who were pushed as a much, much bigger deal and were all 4 better in the ring than him, plus Jericho finally getting his footing in WWE, plus Angle's supernova rise. Taz stood no chance.
Without exaggeration, he was smaller than Crash Holly. He's lucky he didn't get a full blown Oompa Loompa gimmick tbh.