For a long time v8 was the fastest on a set of benchmarks but not on real world work load. That was the time when SpiderMonkey and JSC decided to go on a different path. I believe it was Mozilla that make the move first.

If we believe there is limit to everything, then the only sane conclusion would be v8 and JSC would both perform nearly the same with negligible difference in the long term. So choosing something that is fast, simple to integrate now makes a lot more sense.

Of course that is assuming memory usage, security and others being equal.