The main reason was balance. They wanted pvp balance to be focused around arenas so I guess they figured it would be easier if arena was the only ranked format. The other was population. They probably thought the split between the two ranks would be too much.
Personally I feel they should of left both in and let the community decide and then go from there instead of straight up removing it, but sadly at this point it would be too late to bring 8v8 back.
I will point out that ranked 8v8 pretty much existed on only two servers. Additionally with the exception of huttball none of the 8v8 maps every really worked well in the uber competitive ranked. 8v8 was ridiculously and obscenely cookie-cutter strategy all over. Not to mention the fact that 99% of the time whoever capped the first node wins.