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.