Spring and Spring Boot are incredibly productive and give you a ton of stuff out of the box, whereas some people actually enjoy writing a lot of plumbing code as a distraction/challenge from boring business code. Those people will migrate to "lean" frameworks because it will give them the opportunity to write more low-level plumbing code.
I highly doubt that.
The real reason is likely more about the GraalVM, which spring hasn't supported until very recently. (And still only with caveats)
The people jumping to Quarkus or Micronaut are more eager to chase the new shiny and are willing to spend the time debugging not-yet-mature stacks.
Edit: To be clear, the reply mentions fanboyism but we are talking about the maturity of a stack. Spring has been around for 20 years and is not going anywhere. Quarkus just reached 5.
Especially considering that Springs support isn't full yet, and quarkus has been around for 5yrs now.
They're both production ready stacks though, really strange to have spring fanboyism in 2024
I have also some nice things to say, and I'm not going to make a detailed analysis here but IMHO, it's really not worth the tradeoff.
Personally the experience with JSF and Web Forms, is what makes me appreciate any framework that exposes the underlying browser stack, instead of pretending it is something it is not.