One reason is mdma is such a different and novel state we don't experience often, we're overwhelmed or caught off guard by the feeling that it distracts us from stuttering. This is similar to how if you're really excited or in some state of euphoria from something (achieving some major accomplishment, watching your team win a championship). You're so caught up you're not thinking/worrying about speech. The other change I assume is a chemical one that someone better suited than me can probably answer. But you can't go through life continually rolling in an effort to ameliorate stuttering.