There are a few reasons I can think of. I'll be upfront about the iffiest of them. I philosophically don't like the notion of digital scarcity. I've blogged about it [1] before, but in short, capitalism is a pretty good system for dealing with scarcity in the physical world, but like any system, it doesn't work well for everyone. Cryptocurrencies can allow those who have been failed by the existing capitalist system to improve or worsen their situation, but they do so by introducing digital scarcity, which doesn't make more sense. Increasing the number of capitalistic systems on the Internet reinforces the capitalistic systems in the physical world, and I believe the flaws of capitalism can't be solved by more capitalism in the long term.

Secondly, renewables have been falling in price long before cryptocurrency mining picked up. The economics of renewables don't need the help.

But more realistically, as I alluded to upthread, blockchains don't only run on renewable spikes. Even if they ran on 100% renewables, that would mean solar panels, dams, and wind/wave farms would be rolled out specifically for mining. There's also the mining rigs that are constructed. That equals rare earth mineral mining, manufacturing, and transportation of resources for the sole purpose of cryptocurrency mining, and those actions all have environmental consequences.

When those consequences are in furtherance of something that I think is a net negative for society, I'd rather have the resources used for something else.

[1] https://meipouchou.com/blog/web3-metaverse-lain/