I suppose it's no surprise why the mitzvah of tzitzit (that is, wearing a tallit) is one that has been taken up by women, but tefillin, for the most part, hasn't. At least in part, it's because most people only come to shul on shabbat (if they come at all) and so never see tefillin put on - even by men, let alone by women. But in truth, the mitzvah of tefilin is more important: tzitzit one must only wear if one wears a garment with four corners; tefillin must be worn on weekdays, period.
I'd love to see tefillin become a more regarded mitzvah. I have all these great metaphors which I love to use to talk about tefillin: the tefillin as vine: one in which we are the rtellis, but also in which the vine supports the trellis, so to speak. The batim are fruit, with jeweled words, like pomegranates.
Or else tefillin as compass (בשם אמרו My friend Rabbi Scott Slarskey taught me that one).
And the gemara brings support from today's daf (Berachot 14b):
‘Ulla said: If one recites the Shema’ without tefillin it is as if he bore false witness against himself.16 R. Hiyya b. Abba said in the name of R. Johanan: It is as if he offered a burnt-offering without a meal-offering and a sacrifice without drink-offering.
R. Johanan also said: If one desires to accept upon himself the yoke of the kingdom of heaven in the most complete manner (cont. 15a), he should consult nature and wash his hands and put on tefillin and recite the Shema’ and say the tefillah: this is the complete acknowledgment of the kingdom of heaven. R. Hiyya b. Abba said in the name of R. Johanan: If one consults nature and washes his hands and puts on tefillin and recites the Shema’ and says the tefillah, Scripture accounts it to him as if he had built an altar and offered a sacrifice upon it, as it is written, I will wash my hands in innocency and I will compass Thine altar, O Lord. Said Raba to him: Does not your honour think that it is as if he had bathed himself, since it is written, I will wash in purity and it is not written, ‘I will wash my hands’.