Reddit reply in r/AcademicQuran: how were the sects of islamic theology formed — September 24, 2024 · lopoail | Ali Shukri Amin

I think we're conflating proto-Sunnism and Sunnism proper. Proto-Sunnism's components and tools were there even at the time of the Prophet, but especially after his death. Arguably, so was proto-Kharijism. But since Kharijism is a much less robust religious phenomenon, not only do we see it's roots later in the development of the first community of Muslims, it requires less to develop.

Kharijism as a distinct group was there before Sunnism. That's because Sunnism wasn't a thing until it was defined by not being khariji, Mutazili, qadiri, Shi'i, etc. But proto-Sunnism is what Kharijis were opposing in the first place.

The opposition to Uthman did indeed include proto-Shia but also Khawarij. There was a time when their interests were aligned. However, they didn't become a distinct political movement until they were distinguished from Ali's supporters during his reign and their insurrection against him. There were quite a few factors that lead to the uprising against Uthman, and part of it was the khariji political and religious ideology gaining ground.

Remember that one of the core issues the khawarij advocated for was a meritocrical approrach to leadership. Shi'a advocated for hereditary leadership within the Prophetic family. Sunnis advocated for tribal leadership within Quraysh. Khawarij wanted whoever was best suited for the job, even if they weren't Ahl al-Bayt or even from Quraysh.

Uthman elected many of his family members to positions within government including over lands where they were foreigners. To his detractors, this was blatant nepotism. It was also squarely against the philosophy of the khawarij. (Umar was vehemently opposed to electing family to positions of power)

Sunnism was the winning sect and not the earliest. However, Sunnism's advantage was that it used proto-Sunnism - the oldest, most widely accepted, and popular ideas from the time of the Prophet himself. Any group could have capitalized on those ideas, but Sunnis made the best use of them and tied themselves as much as possible to the early community - even if they disagreed widely within themselves.