Also because the Bahamian government objected. The campaign finance charges were not part of the initial extradition request for SBF, so it was a sovereignty issue. The US prosecutors didn't press the matter, because they had him nailed for so many other charges, just as you say.

Salame was not extradited, so in his case, the US prosecutors could proceed with the campaign finance charges. That process ended with a guilty plea rather than a trial.

People will speculate about a conspiracy to keep the campaign stuff away from trial, but it all seems pretty routine to me.