The Torah declares that before Matan Torah, the firstborns were responsible for bringing sacrifices and Cohanim replaced them after the Golden Calf incident (Num 3,40):
:וְלָקַחְתָּ אֶת־הַלְוִיִּם לִי אֲנִי ה' תַּחַת כָּל־בְּכֹר בִּבְנֵי יִשְׂרָאֵל
and take the Levites for Me, the LORD, in place of every first-born among the Israelite people...
As Rashi explains on Ber 25:31:
בכרתך. לְפִי שֶׁהָעֲבוֹדָה בַּבְּכוֹרוֹת, אָמַר יַעֲקֹב אֵין רָשָׁע זֶה כְדַאי שֶׁיַּקְרִיב לְהַקָּבָּ"ה:
THY BIRTHRIGHT — Because the sacrificial service was then carried out by the first-born sons, ...
Why Abel brought his sacrifice on his own and didn't ask to do that from his firstborn brother?