I wouldn't exactly say that AppEngine doesn't have much traction. Every month, half the world's IP addresses touch some service hosted on AppEngine[1], which is a pretty huge number.

[1]: And that statistic is more than a year old now (from 2012's Google I/O: http://gigaom.com/2012/06/28/google-app-engine-by-the-number...), it's a higher number now.