I don't understand why there's still 1 whole exahash mining on the BIP110 endpoint
Because when miners have some hardware, then they have to mine something. If the new algorithm won't be successful, then it may be needed to switch to the old algorithm, and maybe adjust the difficulty.
Some fork, following BIP-110 rules, was already made:
Bitcoin Purity:
https://mempool.bitcoinpurity.org/BIP-110:
https://mempool.guide/And now, there are at least two chains with BIP-110 rules.
forking to an altcoin with an entirely different algo
What if that chain will be less successful, than the original BIP-110? Or what if Bitcoin Purity will have higher value, than "Bitcoin Blake"? It is P2P world: Luke can be a leader for some people, but if he will release something, that will be buggy, and will halt again, just like BIP-110 halted, or will not meet users' needs for whatever reason, then other people could switch to a different chain. Basically, the position of "BIP-110 leader" is not granted automatically: you need to have a successful chain, to be a good leader, otherwise people will follow someone else, because another chain can deliver faster a working product.
Some people are really eager to waste hash power on a network that is destined to be useless and mines a single block every few handfuls of hours.
When your coins are hard to made, then they are more resistant to the spam, and they are usually worth more. If BIP-110 original chain will be successfully unlocked, for example by using fractional confirmations, and weak blocks, then it can be more successful than the competition, just because of following the original rules.