The pasuq (still in parashas Re’eih) discusses the various species of kosher and non-kosher mammals (Devarim 14:3-8), marine animals (v. 9-10), and flying creatures (v. 11-20). And the terminology is that this species is tamei, whereas that is tahor.

Then, in the final pasuq (14:21):

לֹא תֹאכְלוּ כָל נְבֵלָה, לַגֵּר אֲשֶׁר בִּשְׁעָרֶיךָ תִּתְּנֶנָּה וַאֲכָלָהּ, אוֹ מָכֹר לְנָכְרִי, כִּי עַם קָדוֹשׁ אַתָּה, לַה’ אֱלֹקיךָ; לֹא תְבַשֵּׁל גְּדִי בַּחֲלֵב אִמּוֹ.  {פ}

Do not eat from anything which died on its own; give it to the stranger who is in your gates, so that he can eat it, or you my sell it to a foreigner; because you are a holy  nation to Hashem your G-d; do not cook a kid in its mother’s milk.

There is a shift. When discussing which animals we may or may not eat, the Torah frames it in terms of tamei vs. tahor. But when describing the requirements of shechitah and separating meat and milk, the attribute in question is our qedushah.

As I wrote in a comment on the haqdamah to Shaarei Yosher:

When the Torah speaks of taharah,  the proposition is “mi-”, from, e.g. “vetiharo min hatzora’as”– and he [the kohein] purifies him from the [tum’ah associated with the spirito-somatic illness,] tzora’as“. What is taharah? While many object to translating it as “spiritual purity”, the word is used to describe the “pure gold” of the menorah, “zahav tahor”. Taharah is freeing the soul from a kind of adulteration, just as it describes gold that is free of impurities.  The tahor soul is one that is free from the habits and effects of living within an animal body.

On the other hand, qedushah is about pull. The golden tzitz on the kohein gadol’s forehead reads “qadesh Lashem”. Qedushah is being set aside for a given purpose. The wedding formula, “Hereby you are mequdeshes li…, committed to me…” uses the term where the “to” isn’t Hashem’ purpose. But in usual usage, if the “le-” is not provided, it means creation’s Ultimate Purpose, “for My Honor, lekhvodi, I have created it”.

(And then continued in R’ Shim’on’s vain that the Ultimate Purpose is to partner with HQBH to share His Good with the rest of creation.)

This dichotomy may explain the shift here as well. When the subject is what we eat, avoiding certain foods is staying away from what’s wrong, and thus is a path to taharah. But having rituals for how we eat, that elevates eating into a means of remembering our higher calling and serving him, and thus is part of striving for qedushah.