I might just have missed it, but I don't remember that ever being explained, retconned, origin-storied or prequeled during the run of the original series... a bit fuzzy on the movies, most of them I can't properly keep apart in my memory banks.
One thing I know is this: I did not like the Star Wars reboot too much, I thought it had too much action, too much blah and quite a nonsensical story. But I did not outright hate it, either. I thought this could go somewhere.
Enter Into Darkness.
The franchise has just left the building, stark naked and screaming.
I still like the original series. I do remember some choice bits I found inspiring and tolerable about TNG. DS9 is mostly just single scenes and images, the whole thing annoyed the pants off of me. The origins TV series... I was looking forward to that, forgot all about it after... months? weeks?
Into Darkness made me not bother about the franchise any longer. I give up. I'm pretty pissed off. Why can't people come up with their own franchises to ruin?
Also, complete waste of the Cumberbatch. Bastards.