After Viviz (SinB, Eunha, Umji) unit announcement we kind of get things clear here. (in different agency)
Sowon and Yerin wanted to pursue acting careers. Yuju probably wanted to mix group activities with solo debut.
When GFriend disbanded I've already said that if 2-3 members didn't want to continue as full time Idols there wasn't any financial sense of Source to renew group contract, there also wasn't sense to continue with just 4 members. Because with renewal Source Music would need to give girls better deal, so more money for members, less for agency but...
if we think that GFriend sales were already going down, and without 2 members many Buddies will leave and don't buy albums anymore then GFriend wouldn't be a 'source' of income for Source Music. I would even say that with growing MV production costs Source would need to go even more in debt to be able to promote group well...