They're both calls to action: avoid cyberpunk, seek solarpunk. We've gotten so much "avoid this" fiction in the past decades that people are just burnt out and actual visions of a positive future have been largely absent since Star Trek, so it makes sense to say "hey, you know all these abstract political ideas that we talk about? this is what the world might look like if we implemented them". Nothing about solarpunk says that it is the default outcome. No reasonable person thinks that it is. It's an ideal to fight for.

The era of collapse fiction like cyberpunk has only really given rise to one political movement: accelerationism. Whose defining trait is "if the world's going to collapse, we should probably hurry it along so we can see if there's anything to salvage afterwards".