It was very important to us to have a gradual and smooth upgrade path, such that you can start using Hack by just tweaking the "<?php" header at the top of a source file.

Obviously switching to a syntactically incompatible programming language would be at odds with that goal.

Haxe is much more structured in how it's organized. It requires an explicit class for code, it has very few methods or variables in the global namespace, it has block-level vs. function-level scoping, etc. Those are pretty typical language features for languages designed for large code bases, but they're probably unfamiliar territory for some php developers.