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.
If they wanted balance to center around arenas, why aren't they balancing around arenas o.O
Oh that's right cause there is no dev team.