I have no idea why they decided but my guess is that * XML and JSON can be slow. * XML parser are big * XML file size is big * JSON lacks proper binary supp

I have no idea why they decided but my guess is that * XML and JSON can be slow. * XML parser are big * XML file size is big * JSON lacks proper binary support * Thrift defines a entire transport stack, so if you already have you already have a transport layer, you are adding more bloat * Some people may find protocol buffers and thrift files too hard/tedious * Why not? Sometimes it's easier/more fun to roll out your solution