ISS is not much more than a big set of maintenance contracts in search of a reason to justify its existence.
Before shuttle retired, the US flew up a lot of extra equipment to handle future repairs. Dragon is going to shift a lot of the taxi service fees to the US. Russia just doesn’t want to pay it’s share of the maintenance bill and the russian contractor can make more money with less international oversight building its own modules.
I used to joke, somewhat unfairly, that the purpose of the Space Shuttle was to send stuff to the I.S.S. and the purpose of the I.S.S. was to give the Space Shuttle something to do.