The problem? He was never placed as any kind of threat to either The Rock or to Austin. Austin pretty much destroyed him to the point that Austin was arrested for trying to almost kill Rikishi. Rikishi wasn't anywhere in The Rock's league, he did it for The Rock, and had to use a car to do it, essentially presenting himself as nothing but a coward and showing that The Rock was above him, simply because of the means he had to go about doing it.
The crowd, as memory serves, despised it, it did nothing for Rikishi. It split up one of the most over acts in the company in the process. It went over so badly that they eventually had to turn attention back onto HHH (who was originally suspected of being the one that did it, had every motive to do it, and was the most obvious person to do it).
It was a swerve, but a poorly executed swerve, they knew everyone would think HHH was involved, Rock was cleared of any involvement (in the people's eyes, nobody wanted to hate The Rock at this time), so you had two options. Go with the most obvious choice in Triple H or find someone that has a connection to someone in the event and use him. Hence the, "I did it for the Rock" and the "Great White Hope" motive.
It was just a bad swerve that didn't really do anything for anyone, and it was something that did hurt Rikishi in the process. If they'd just gone with the obvious choice of HHH, Rikishi would have been fine and wouldn't have lost momentum, because they didn't, Rikishi was still over when he turned face again, but he never was quite as over as he was before that point.
I'd also like to add. Remember who the target was, it was Stone Cold Steve Austin, and this was the angle that took him out for an extended period of time. Whoever did it, they wanted Austin to get revenge on them, and it was even better if the crowd hated them for it. Nobody wanted to hate Rikishi, he was a beloved babyface, he was over with the crowd, they loved the guy, but HHH? How could they not hate this guy and how could the crowd not love seeing Austin get revenge on HHH, and they did, they loved every moment of Austin beating the hell out of him.
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