Probably because it's largely irrelevant to the matter at hand - Belarus is not the country getting invaded, and Ukraine is not the one that violated the agreem

Probably because it's largely irrelevant to the matter at hand - Belarus is not the country getting invaded, and Ukraine is not the one that violated the agreement so there's zero justification for an invasion from either Russia or Belarus. If anything, highlighting Belarus's involvement in the invasion serves to demonstrate that Russia's meddling in Belarus's elections was to install a puppet government and is a violation of their independence (and subsequently the memorandum).