פירוש על קידושין 67:7

The Shema also mentions the mezuzah, and women are obligated in mezuzah. So why not say that tefillin are compared to mezuzah and that women should be obligated to wear them.
The answer is that tefillin is immediately juxtaposed to the study of Torah in both the first and second paragraphs of the Shema, but it is juxtaposed to mezuzah only in the first. Therefore tefillin is more like Talmud Torah than mezuzah.