(8) And you shall explain to your child on that day, ‘It is because of what ה׳ did for me when I went free from Egypt.’ (9) “And this shall serve you as a sign on your hand and as a reminder on your forehead—in order that the Teaching of ה׳ may be in your mouth—that with a mighty hand ה׳ freed you from Egypt.
(6) Take to heart these instructions with which I charge you this day. (7) Impress them upon your children. Recite them when you stay at home and when you are away, when you lie down and when you get up. (8) Bind them as a sign on your hand and let them serve as a symbol on your forehead;
And thou shalt fasten them for a sign upon thy hand, and it shall be immoveable before thine eyes.
Do not forsake your mother’s teaching. (21) Tie them over your heart always;
Bind them around your throat. (22) When you walk it will lead you;
When you lie down it will watch over you;
And when you are awake it will talk with you.
In the ancient world, women were associated with the home. One might surmise that women and infants, as well as males and older children (once night fell), would have been considered within the domain of the mezuzah more often than not, where there would have been no need for the protective function of tefillin ... The blanket exclusion of Shabbat tefillin practice can be explained in similar fashion. When at home, in the domain of the mezuzah, there would have been little reason for people to wear tefillin.
Rebbi Yannai said: Tefillin need a clean body. Why does one not hold on to them? Because of the impostors. It happened that a man deposited something with another, and the latter then reneged on it. He said to him: Not you I thought trustworthy but those on your head I thought trustworthy.
כאלישע בעל כנפים - להכי נקט אלישע דמסתמא כיון שנעשה לו נס בתפילין היה זהיר בהן והיה לו גוף נקי ומזה הטעם אין אנו יכולין ליפטר שהרי בזה אנו יכולין להזהר ואין תימה על מה שמצוה זאת רפויה בידינו שגם בימי חכמים היתה רפויה
[L]ike that of Elisha, Man of Wings - Thus Elisha holds that because a miracle was done for him with tefilin, he would be careful with them and would make his body clean. For this reason we cannot get rid [of this mitzvah] because we could [also] be careful. It is not surprising that this mitzvah is lax in our hands because also in the days of the sages it was lax.
וּמָה שֶׁאָמְרוּ (שבת מט, א) שֶׁתְּפִלִּין צְרִיכִין גּוּף נָקִי, וְאָמְרוּ בַּגְּמָרָא מַאי גּוּף נָקִי? שֶׁיִּזָּהֵר שֶׁלֹּא יָפִיחַ בָּהֶן, אֲבָל אֵין הָעִנְיָן לוֹמַר, שֶׁצְּרִיכִין גּוּף נָקִי מֵעֲבֵרוֹת אוֹ מִטֻּמְאָה, כִּי כָּל אָדָם וַאֲפִלּוּ טָמֵא וּבַעַל עֲבֵרוֹת מְחֻיָּב בְּמִצְוַת תְּפִלִּין, וּבִלְבַד שֶׁיֵּדַע לְהִזָּהֵר שֶׁלֹּא יָפִיחַ בָּהֶן, וְאוּלַי מִתּוֹךְ הַתְמָדָתוֹ בְּמִצְוַת תְּפִלִּין שֶׁהֵן זִכְרוֹן גָּדוֹל לָאָדָם בִּמְלֶאכֶת שָׁמַיִם יָשׁוּב מִדַּרְכּוֹ הָרָעָה וְיִטְהַר מִכָּל גִּלּוּלָיו ... וְהַמַּחְמִירִים בִּקְדֻשַּׁת הַמִּצְוָה וּמְנִיאִים לֵב הֶהָמוֹן בְּדִבְרֵיהֶם מֵהִתְעַסֵּק בָּהּ, אוּלַי כַּוָּנָתָם לְטוֹבָה, אֲבָל בֶּאֱמֶת יֵשׁ בָּזֶה מְנִיעָה לִבְנֵי אָדָם בְּכַמָּה מִצְוֹת וְהִיא רָעָה רַבָּה.
And [also] that which they said (Shabbat 49a) that tefillin requires a clean body, and they said in the Gemara, “What is a clean body? That he be careful not to pass gas with them [on].” But the matter is not to say that it requires a body clean of sins or of impurity. As every man, even one impure or a sinner is obligated about the commandment of tefillin, so long as he knows to be careful not to pass gas with them [on]. And maybe from being constant with the commandment of tefillin — as they are a great memory device for a person about his service to the Heavens — he will repent from his evil way and purify himself from all of his filth ... And maybe the intention of those that are stringent about the holiness of the commandment, and discourage the hearts of the masses with their words from being involved with it, is good. But in truth, through this, there is the prevention of people from several commandments, and [so] it is a great evil.
גדולה מצות תפילין שכל המניחן מאריך ימים ... וכל מי שאינו מניחן הוא בכלל פושעי ישראל בגופן לכך צריך כל אדם להיות זריז בהן: מצותן להיותן עליו כל היום אבל מפני שצריכין גוף נקי שלא יפיח בהם וצריך שלא יסיח דעתו מהן בעודן עליו ואין כל אדם יכול ליזהר בהן על כן נהגו שלא להניחם כל היום ומ"מ צריך כל אדם ליזהר בהן להניחן בשעת ק"ש ותפלה: קטן היודע לשמור תפילין אביו חייב לקנות לו תפילין לחנכו:
So great is the commandment of tefillin is that whoever puts them on prolongs his days ... And whoever does not put them on is generally considered one of the criminals of Israel with their body. For this reason, every person must be diligent in them. The commandment is to have them on all day long, but because they require a clean body that does not smell with them, and he must not be distracted with them while they are on him, and not every person can be careful about them. Therefore, they are accustomed to not putting them on all day long. Nonetheless, every person must be careful about putting them on during the Shema and prayer. A child who knows how to observe tefillin, his father must buy him tefillin to educate him.
I would suggest that at the exact point in history, just after the fifteenth century ... something soon occurred that changed people's perspectives of mitzvot like Mezuza and particularly Tefilin. This was the advent of the Lurianic Kabbalah of R. Yitzchak Luria, known as the Ari Zal (1534-1572). This new mystical system introduced, or certainly popularised, the notion of theurgy, perhaps more than any previous mystical system had done before. Theurgy is the almost 'magical' notion of 'spiritual cause and effect.' In other words, by the physical observance of the mitzvot, one not only fulfils the legal and clinical requirements of law, but one attains protection and renewed spiritual energy through these very actions ... From that point on, promoters and advocates of religious observances had a far easier time than the earlier Gaonim who only had technical legal precedent to work on. The Gaonim could travel to Spain to promote Tefilin observance amongst the masses, and author responsa literature, but to no practical avail. This all changed after Lurianic Kabbalah.