Its EA. They were scared that a PC version that included CoOp superior to the shitty console edition may cut into there profits. Why do you think they delayed the already finished version for the PC to have a simultanious launch with the consoles? More time to polish it my ass.
No, it wasn't finished. Bioware never said it was finished. They have squashed that statement multiple times on their own forums. The on-disc content was finalized, meaning they weren't going to add any more to the disc. That does NOT mean the game was ready to ship. The game would have been delayed either way and they HAVE been working on the PC version and extra content since that time.