However, in Babel 6 there were a few (nice) incompatible changes to the AST which was unfortunate for interop and other tools (https://github.com/babel/babel-eslint had to make a lot of changes).
I don't want to speak for him, but I think a lot of the decisions to fork (parser, etc) at the time were most likely made like mentioned above to move faster. And at the time, Sebastian was basically the only one working on it. Maybe more context from his post: https://medium.com/@sebmck/2015-in-review-51ac7035e272#.qcyz...
Yeah there's multiple JavaScript parsers: esprima, acorn, shift, babylon (uglify has their own parser I think).
There was an effort to standardize as well in https://github.com/estree/estree.
EDIT:
However, in Babel 6 there were a few (nice) incompatible changes to the AST which was unfortunate for interop and other tools (https://github.com/babel/babel-eslint had to make a lot of changes).
I don't want to speak for him, but I think a lot of the decisions to fork (parser, etc) at the time were most likely made like mentioned above to move faster. And at the time, Sebastian was basically the only one working on it. Maybe more context from his post: https://medium.com/@sebmck/2015-in-review-51ac7035e272#.qcyz...