We also have 80 years of experience with the flu vaccine opposed to 8 months
8 months is incorrect. Here's[1] an article from 17 months ago about results from Moderna's covid-19 trials for vaccinations dating back to mar16 2020. So it's been 21 months, not 8 (and that's ignoring trials of mRNA vaccines years earlier as they weren't for this specific virus)
[1] https://www.cidrap.umn.edu/news-perspective/2020/07/hopeful-...
This is the answer for why not all annual flu vaccines need clinical trials.
Which one?
Here is a pretty good summary of the history of vaccine development.
https://www.medscape.com/viewarticle/812621_1
While nearly all influenza vaccines generate similar antibodies, Covid and influenza vaccines generate significantly different antibodies
Influenza vaccines generate antibodies against influenza Hemagglutinin proteins targeting sialic acid receptors
Covid vaccines generate antibodies for (S) glycoproteins targeting ACE-2 receptors.
In short, we have 80+ years of experience generating antibodies for hemagglutinin, and much less for (S) glycoproteins.