http://revach.net/article.php?id=3352

Section: Taryag Mitzvos   Category: Insight
Chasam Sofer: Women Did Leave Mitzrayim, But Are Not Obligated To Wear Tefilin

The Torah says that Tefilin remind us that Hashem took us out of Mizrayim.  If so women should also wear Tefilin even if the mitzva is time dependant because "Af hem hayu b'oso hanes", they were also part of Yetzias Mitzrayim.  This same principal is used to obligate women in other mitzvos that they would otherwise be exempt from.

The Chasam Sofer answers that we must distinguish between Matza for example, where the undelying reason for the mitzva is to remember Yetzias Mitzrayim, and Tefilin where the underlying reason is to show our love for Hashem by wearing His signature on us.  As a natural consequence of our thinking about our love for Hashem we remember how he took us out of Mitzrayim in grand fashion.  However this is only an outgrowth of the main reason.  Since the main reason is not Yetzias Mitzrayim, this secondary reason does not obligate a women in the mitzva of Tefilin.