Emperor Norton wrote:
The real answer is that ENT started off with an amazingly interesting idea, but didn't know what it was doing and ended up bringing in TNG races for no other reasons that a lazy demographics grab.
Hence, why were there Borg? Because they wanted an excuse to have Borg. There was no reason for it, since the 22nd century could have been interesting all on its own (also why there was no reason to tie the series to time travel).
You can create any in-universe explanation for it, and lord knows we have to, but that's the actual reason.
Really? The Borg were in ONE episode. And it fits in very well with the rest of the series. And ENT didn't bring in a bunch of TNG races. The main players throughout the show were the Vulcans, the Andorians and the Klingons, all established in TOS. And they did a huge story arc in season 3 with a race that had never appeared before at all.