My understanding is that the original engine was simply too resource intensive - even the best PCs struggled to run it optimally.

This meant that the game was somewhat future-proofed and looked absolutely incredible on a high-end PC, but it also meant the opposite was true - that most players would have a hard time running the game at all, especially PS3 users (the PS3 simply did not have the RAM to handle to the original engine which is why it got stalled for so very long).