If the high holidays are really so momentous and paramount in Judaism, why are they not even mentioned in passing in the Torah until far into Leviticus, in fact, after the mishpatim, pilgrimage holidays and many other precepts?
The "High Holidays" are certainly momentous, but it's not necessarily the case that they're of paramount importance in Judaism, compared to all of the other holidays, laws, and lore. One could argue that Passover is a more important holiday, as evidenced (in part) by the fact that we number the months of our calendar starting with the month Passover occurs in. Or one could argue that the weekly Sabbath is the holiest day, since it has the most Torah readings and the severest penalties for its violation. Rosh Hashana and Yom Kipur each certainly have unique importance and solemnity, and deservedly get a great deal of attention in the Jewish world, but they don't necessarily outrank everything else.
It's pretty clear that Judaism doesn't have a single observance that truly rises above all others in importance. Torah study, living in the land of Israel, the Sabbath, mila, tzitzit, and acts of kindess are each described in the Talmud as having weight equal to all other commandments put together. In fact, R' Yehuda Hanasi admonishes us explicitly in Avot 2:1 not to be to confident about intuitive rankings:
וֶהֱוֵי זָהִיר בְּמִצְוָה קַלָּה כְבַחֲמוּרָה, שֶׁאֵין אַתָּה יוֹדֵעַ מַתַּן שְׂכָרָן שֶׁל מִצְוֹת.
And be as careful with a light commandment as with a weighty one, for you do not know the reward given [for the fulfillment] of [the respective] commandments.
Finally, it's not necessarily the case that concepts are presented in the Torah in order of their importance. The aforementioned commandment of Torah study doesn't come until Deuteronomy. It's hard to say exactly what God's purpose was in putting the Torah in the precise order that He did, but suffice it to say that with its mix of narrative, values, and laws, the order is definitely more complicated than "most important first."
If you're interested in learning more about the grand complexity of Jewish observance, I encourage you to click through some or all of the links presented here and keep digging.
Adding to what Isaac Moses wrote, the section and focus of what is in Leviticus (Emor)is the only place that describes all the holidays. Well, Pinchas doesn't focus as much on a "description" but more on the sacrifices to be offered.
The other places that holidays are mentioned such as in R'eh and Mishpatim are focused specifically on the 3 Pilgrimage festivals. Thus Rosh Hashanna and Yom Kippur and Shabbat are not mentioned, there.