Weren't the Yugoslavs pressured a bit to give the Croats that level of autonomy? Or it may have been that once they did everyone realized Yugoslavis was done for (or thought it would be). If there was going to be a Greater Serbia it would probably not give as much Serb occupied land to the Croatians,
The 'Yugoslavs' were not pressured, the Royal-led goverment was.
BTW - the Banovinas were tailored in such a way by a victorious (= Royal Serb) side so 6 of them will have Serb majority despite many of Macedonians, Montenegrins, Albanians, Moslems, Croats and Hungarian being there. Of other three, one went to Slovenians (by mostly hundred years dividing line towards Croatia), while two went to the Croats (mostly by historical borders vs. Ottoman Empire plus Western Herzegovina).
It would've been an interesting time line with Serbs saying to themselves - okay, with these 6 Banovinas we have gathered 99% of the Serbs (Royal Serbs counted as Serbs also the Macedonians, Montenegrins, plus Moslems with Slavic roots), so let's ditch Croatia and Slovenia for good - we have actually reached the Greater Serbia goal, and we don't have to listen the demands from Zagreb and Ljubljana any more.