in case I'm wrong GTA Online takes place after the events of GTA V and Rockstar wants GTA Online to be it's own little thing
I thought I read somewhere that GTA Online takes place before the events of the story. I'm really not sure either.
OT: The only official explanation I've seen is the "the game is so massive, we want players to get used to everything so they're not overwhelmed" one. I don't remember the exact quote or the source.
Personally I think it's BS and the delay is for technical issues (server stability, etc.).
It's a mix of both.
Truly, it is for technical reasons, hence the beta testing going on now.
However, it also does help a player get used to everything. Hell, after story and freaks and strangers, I've been practicing my driving/shooting/etc for GTA Online. So I can get used to everything, learn the best areas, and learn how to dominate the stock market.