While I can't comment on why most of the people who played SWTOR left, I will say this: my computer had barely enough ram to run the game, and I had to deal with constant hitching as the game suffered from page fault after page fault. I would have had to buy a totally new motherboard/cpu/ram to solve this problem. I would have spent the money to buy it, if the game had been compelling or interesting enough. Sadly, after about 3 months, I found my interest flagging, and eventually stopped logging in, except to raid. When I mentioned my intentions to quit at the end of my subscription time, most of my operations group agreed and raiding collapsed.
This game is too much like WoW. And I played WoW for 7 years. It just gets boring after a while. I want something new.