Mainly to get off HPHP and relieve both that team and ours of the constant coordination and toe-mashing. We thought “modernizing” the code would be a better investment than jerry-rigging the Zend engine back into our infrastructure. I think it was the right call.
Also, with Python we could interact with other services using basic Thrift bindings, and even provide our own endpoints. With PHP, all RPCs and queries were prebuilt into our standard libraries with custom glue code that increasingly made non-Zend assumptions. Again, we could have made it work, but we would be swimming against the current.
All the rest of the database management code ended up being moved to Python, and benefitting greatly, so it eventually worked out.
In summary, organizational concerns.