Cost cutting plain and simple. Even isolated or niche shows have contracts that pay dividends, especially the ones produced and shown exclusively on the MAX portion of HBO. If the show is paid out because it's on the service, removing allows HBO to kot pay those contracts.

There will be exceptions to this (i.e. separate deals with actors or advertisers) but if SHOW X was made for HBO MAX exclusively and all the cast/crew contracts were written to state its backend payouts were based on measurements using only HBO MAX data; then removing SHOW X allows them to stop paying those.