This is a good observation, the answer is that the notion of matter shifted away from the Aristotelian one around the time of Aquinas, and he was instrumental in effecting the change. According to the Aristotelian doctrine, matter was indeed the expression of any potentiality, any capacity for change. This led to a hierarchy of "spiritual matters", and form-matter composites turning into matter for higher order composites (in Aristotle's Physics, book II wood is a composite of matter with "woodenness", but at the same time it serves as mere matter for a bed).
To summarize the shift: yes, all change requires potentiality, but not all potentiality is material potentiality, that is the provenance of corporeal things subject to locality and motion. One can hear a distant echo of this shift in the idea of extension as the chief attribute of matter in Descartes and Spinoza. For spiritual "pure forms", like angels, Aquinas replaces matter as the principle of individuation and change with esse, the (act of) being, the third ontological ingredient he adds to the Aristotelian matter and form (a good short exposition is Maurer's article in A History Of Philosophical Systems).
Pasnau gives some history in Form and Matter:
"This conception of matter lends itself naturally to universal hylomorphism:
the doctrine that every (created) substance is a composite of form and matter... Many early scholastic authors, especially Franciscans, embraced this sort of
view. From the time of Albert the Great and Thomas Aquinas, however, the
view fell entirely out of favor, replaced by the idea that matter occurs only in the corporeal realm. Hence there arose the linkage we take for granted today,
between corporeality and materiality, so that to be a body (corpus) is just to have matter."
Aquinas wholeheartedly embraces interpreting matter as potentiality, but only as long as one is willing to make distinctions between different kinds of potentiality. And his preference is to avoid the "equivocation" altogether by restricting "material" to the tangible, corporeal. Kenny discusses the issue in detail in Aquinas on Mind, Ch.11. Here is Aquinas himself in Quaestiones Disputate de Anima:
"...to receive, to be a subject, and other things of this sort, are not found in the soul and in prime matter in the same specific way. For prime matter is actuated by means of change and motion, and since every change and motion may be reduced to local motion, as the primary and most universal type of motion, as is proved in the Physics [VIII, 7, 260b 6], it follows that matter is present only in those things in which there is potency to place (ab ubi). Moreover, things of this kind, which are circumscribed by place alone, are corporeal. Hence, in accordance with the way in which the philosophers have spoken about matter, matter is present only in corporeal things; unless, of course, someone wishes to employ matter in an equivocal sense."
We can only speculate why the shift occurred. Perhaps, it made for a cleaner separation between the lowly and the spiritual than malleable Aristotelian "matters", and/or "souls as pure forms" were more credibly incorruptible than the ones made of "spiritual matter". Aquinas does draw a distinction between the latter and angels ("and the very fact that the soul in a certain way requires the body for its operation, proves that the soul is endowed with a grade of intellectuality inferior to that of an angel, who is not united to a body"), but it is moot for the present purposes since for him even the human soul can subsist without a body. Aquinas personally also had additional reasons related to perception of universals in his theory of cognition. Here is from Summa Theologica:
"The intellectual soul, therefore, is pure form, not something composed of matter and form. For if the intellectual soul were composed of matter and form, the forms of things would be received in it as individual. In that case it would not have knowledge of anything except singulars. [...] Now the receptive potentiality in the intellectual soul is other than the receptive potentiality of first matter, as appears from the diversity of the things received by each. For primary matter receives individual forms; whereas the intelligence receives absolute forms. Hence the existence of such a potentiality in the intellectual soul does not prove that the soul is composed of matter and form. [...] In intellectual substances there is a compounding of actuality and potentiality, but not of matter and form, but of form and the being in which it shares."
Replacing "spiritual matter" with esse may have worked for souls and angels, but interpreting the non-composite ("prime") matter as "the" material of the corporeal realm, while retaining its "pure potentiality", led to quaint conclusions about the said material. It appeared to be devoid of any content. Duns Scotus, and more forcefully Ockham, took this position to task, and completed the actualization of (now corporeal) matter. From Ockham's Summula Philosophiae Naturalis:
"That matter is a certain actual entity is clear, because that which does not exist can be a part or principle of no being. But matter actually is a part and principle of a composite being. Therefore it is actually an entity in act. Further, every substance is in act in the natural world. But matter is a substance, since it is a part and principle of substance, and that which is a substance is composed only of substances, according to the Philosopher. Therefore matter is truly in actuality."