It could be because he is still embroiled in legal challenges from when he was President, and a significant portion of voters believe that he is still the legitimate US President...

https://www.reuters.com/world/us/53-republicans-view-trump-true-us-president-reutersipsos-2021-05-24/

As of a few months ago, 40% of voters thought that Trump lost illegitimately.

https://www.newsweek.com/40-americans-think-2020-election-stolen-days-before-midterms-1756218

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This would explain why he doesn't have much search interest prior to his campaign, despite being a TV celebrity, a famously controversial Tweeter, and a name in the public consciousness for decades prior to 2016.