I think MP was seeking that camaraderie that he was feeling with Transatlantic and Avenged Sevenfold at the time, that he wasn't feeling with DT at the time, but used to. I don't know how popular A7F was/is outside of the US, but DT are/were pretty popular outside the US when Portnoy was in the band so I can't imagine it was all about the money. Sure, A7F had a few hits on US radio, unlike DT, but radio was already on its way out as a place to hear the newest biggest rock hits in the late 2000s. The public image was that JP/JR/MP was the core and they had the camaraderie, but JM was the quiet guy, and there seemed to be some rift between MP and JLB, the lead singer. That's not a great situation to return to after recording and touring with TA and A7F, having a blast, everyone getting along, etc...
Whatever the reasons for leaving, I still feel the band was always better with Portnoy. He kept the band grounded, gave them street 'cred, but towards the end they didn't sound as fresh as they once were, so a hiatus would have been musically beneficial for all involved. Now, it's a bunch of nerds playing nerdy prog-metal with no soul, no offense. It's all technician music. Just not my thing.