Very low learning curve, easy to separate functionality rather than relying on some mega AIO Next.js app, easy integration with cloud providers such as Azure, and it's already widely used in the industry (valuable experience to have).
So why aren't start-ups using it more often? It seems like the perfect framework to build with in almost every capacity (unless you really dislike Microsoft).
B) It's not that popular for frontend, which is a very important concern for most early startups
C) Microsoft was historically only used because it was Microsoft, not because it was the best choice
It's mostly a Microsoft brand problem that they rightfully earned by being bullies producing over-complex, stagnant corporate IT trash.
This year, I've switched to Go for all projects moving forward, and will never recommend anyone ever touch anything MS has any part in.