Because of the things that I have seen and heard out of Trump's mouth (like the grab 'em by the pussy comment, as an example). The guy obviously fits the profile of a playboy who does not respect women. So the fact he'd pay a porn star just for the ego trip of screwing her (or to keep quiet after doing so) when married is very believable.
The Russian conspiracy theory has no real proof, and if there was proof of it, you better believe Obama would have presented it before Trump took over. Between all the lies that have come out (the pee pee dossier, for example), it is much more probable that this is manufactured bullshit the Democrats are running with over being ass-hurt in losing the election.
You don't have to see things my way. That's the beauty of free will.