It appears those with a vote regard adoption of PHP 7 as the higher priority and therefore compat with existing code using mcrypt must be ensured. http://news.php.net/php.internals/82191
I haven't used php, but the situation described at that link is codependence.
a: "Most of our users don't care about security."
b: "OK then they can continue using old broken versions."
a: "No, then they won't be secure! Therefore it must be easy to upgrade."
b: "How?"
a: "By not making the proposed security improvements."
This seems like a recipe for losing any users that do care about security, which is not a viable strategy over the long term.