It was basically just a movement that wanted ETH to keep transactions immutable, because after DAO exploit community got divided if actions of that hack should be reversed or not.
Majority went with vitalik, and ETC got basically no developers compared to ETH, and since there aren't enough users, it wouldn't be worth while for anyone to develop it or on it.
Also calling anything ETH killer is just lazy. Just because others exist doesn't mean there's no room for lot of them to exist at the same time. ETH has over 2x marketcap of SOL so i don't think it's even close to overthrown ETH at any time soon. Imho people are just exited because SOL is doing fine in terms of price growth while ETH price just declines slowly. People were predicting it would after PoS, so i don't see the big issue with that.
The majority went with Vitalik because they "idolized" him. Even if his actions were meant to compromise the immutable nature of blockchain tech. Well, they took a wrong decision because now Ethereum is a centralized "shitcoin". Fortunately, Ethereum Classic retains the original vision of Ethereum without compromising decentralization and immutability. Even though ETC is not very popular these days, the Blockchain is still very much alive (healthy). Developers made ETC a scarce cryptocurrency by implementing a hard cap in supply of 210m units. It's as close to Bitcoin as it gets.
So if you're one of the "OGs" who love crypto's original fundamentals/principles (like me), you'd choose ETC over ETH any day. Hopefully, ETC lives alongside BTC for a long time.