Because i want to use my own frontcontroller.
Instead of using the event system and hack around with all the dependencies (request, response, xenforo router, etc...), i prefer to use my own entrypoint.
So all i want to do via htaccess is
if url is:
foobar.com/members => use newindex.php instead of index.php
foobar.com/baz => use newindex.php instead of index.php
foobar.com/bazinga => use newindex.php instead of index.php
everthing else should use the standard xenforo rewrite rules