I think it's because there was a series of coincidences and it's easy to believe.

Trump DID attend a previous dinner and was roasted but he wasn't president at the time. But he is well known for attacking anyone that doesn't praise him openly. So he hasn't traditionally attended because it hurts his little ego. It seems very suspect that the one time he does, he manages to avoid it after all.

Certain members of his staff, seemed to know the name of the attacker with ridiculous speed or even beforehand. (I have not seen time stamps on those).

Certain members of the press remarked on the unusually sparse security. Some even reportedly left because they felt uneasy.

A Fox News reporter seemed to be explaining how the husband of Caroline Leavitt was telling her in advance that she needed to "stay safe" when her audio suddenly and suspiciously cut out. I did listen to that.

The administration pretty quickly spun this as a reason to build TACO's ballroom. Therefore he would benefit from it.

It's easy to jump to this conclusion of these items are true. Personally I doubt it, but when I first heard about it, my first thought was, "How convenient." Let's say if actual proof surfaced I wouldn't be surprised.