What Trump disrespects tends to be predictable: not tough enough, too friendly to foreigners, not conservative enough etc. Putin happens to be tough, conservative and not especially friendly to foreigners, all attributes that Trump respects and yet which are not very common amongst world leaders.
It helps that Putin doesn't speak English and - fantasies by Clintonites aside - isn't actually doing much around the world outside of Syria, a place where they mostly spend their time bombing ISIS (something Trump approves of). So there isn't much reason for Trump to talk about them, beyond the constant Russia-linked attacks on him.
