64bit machines also have a limited amount of memory. It's not just the thread stack size; Apache carries a whole lot of memory per conection. It's much worse when each connection is handled by a Perl/Python/PHP thread.
Apache is generally configured so that the maximum number of _real_ work threads (i.e. including all that state) doesn't cause the system to swap excessively. A slowloris connection eats much fewer resources than that, but Apache doesn't know that and thus reaches the configuration's limit far too quickly.