My understanding is that the delays of DDC were because Geoff Johns hadn't actually decided on an ending when he started writing it. Combined with his priority being on TV/Film, i think it's clear the delays were on him.
As for why they ditched it as the pillar of the DCU going forward, that's 100% down to the delays, whatever their cause. DC simply couldn't hold back on all their publishing initiatives and such for so long, especially it was unclear how long it would take in the end.
Also, I've heard that DC staff have really not gotten along with Johns for a while. It could've been as simply as spite.