There is an obvious reason: Donald Trump paid more money than anyone else in the history of youtube advertising, and so his ads win all the bids and show up in all the places. But it's just been so long now, that they would have been able to catch it and end it, if they really cared. Just figure out where the money enters the system, and block it. If it's more complicated than suspending a single campaign worker's account, then that's newsworthy and should be reported on.

The second explanation is that alphabet is now a card-carrying member of the republican party, and REALLLY wants donald trump to win the next election.

Not an exhaustive list, just trying to brainstorm here.