Astonishingly, many centuries ago few Jews wore tefillin, as the Rashba writes מועטין שבישראל מניחין תפילין, few among the Jewish people put on tefillin. In fact, Tosafos writes that one should not be surprised about the laxity people had concerning tefillin at that time, since they were also in the times of the gemara.
This is shown in many other places, and in the times of the Geonim there are even implications that almost no one in Eretz Yisrael wore tefillin. The Kol Bo, referenced by the Beis Yosef, suggests that in some communities ashes are not placed on a chosson’s forehead because the community members do not wear tefillin. There was even a responsum by R' Sherira Gaon, copied in many medieval works on halacha, answering a question about whether it is yuhara (haughty) for a yeshiva student to wear tefillin when no one else does.
The Semag tells of how he would travel around Europe in the 1200s, preaching to people that they should wear tefillin during shacharis. Even if they cannot control themselves all day, people can certainly maintain a guf naki for tefilla.
Now, let us cite the astounding words of the Meiri (1249-1315). He writes וקצת חכמים נוהגים להניחם בימים שבין ר"ה ליום הכפורים אחר התשובה והטבילה והוודויין וכן הדבר נכון בעיני, there were some sages who wore tefillin only between Rosh Hashana and Yom Kippur after doing teshuva, tevila and vidui, and that is correct in my eyes.
Clearly, the practice of wearing tefillin exclusively during shacharis became prevalent, marking a shift from widespread neglect of the mitzva to minimal observance during shacharis.
Among the reasons offered as to why many people didn’t don tefillin are due to gezeiras shemad (religious persecution) as well as the concern for not having a guf naki.
