that it is regarding flatulence and sleeping
R. Jannai said: Tefillin demand a pure body, like Elisha, the man of wings. What does this mean? — Abaye said: That one must not pass wind while wearing them; Raba said: That one must not sleep in them.
sleeping is easy but flatulence is a problem
says that the reason men do not where teffilin all day is because they do not have a guf noki
and 36.2
not every man can be careful not to pass gas in them
the reason it is called guf noki maybe is because particles of feces getting suck to the body (why is flatulence forbidden in tefillin and during prayer)
i never saw that menstruation has to do with clean body
מוחין כו'. מפני שצריכין גוף נקי ונשים אינם זריזות להזהר אבל אם היו חייבים לא היו פטורין מה"ט דהוי רמי אנפשייהו ומזדהרי כנ"ל דלא כע"ת:
where he says that it is because they are not careful to be watchful and that if they would have been obligated to wear them then this (not being careful of flatulence) will not be enough of a reason not to
but woman are anyway not obligated to wear them
Women, slaves, and minors are exempt from reciting Shema and from Tefillin ...
so better that they should not wear them, then maybe passing gas in them
ps (IMO this is since it is good for women not to be tense but relaxed (it is also better for the children))
i heard that their once was a time that even men stopped wearing teffilin since they where lazy from keeping their body clean (from passing gas), but the rabbis were against this.