Devil's advocate: why were they not successful in developing one against flu, but we're quickly successful when creating one for covid?

There's a well established alternative for flu already? It's like asking why electric engine cars have taken so long to overtake petrol cars; there's a giant existing infrastructure and well known protocols for flu already, so the risk/reward balances aren't great for the newcomer.

Otoh having an established baseline can help understand the efficacy and cost comparison (and time to market), and with new technologies you might want to play it safe?

An emergency like Covid is the ideal environment to dismiss all concerns about efficacy and cost. The risk to losing some money on a failed vaccine is negligible compared to the cost dragging out the restrictions even just a little bit longer than necessary.

Disclaimer: no education in biotechnology or vaccines, just an observer who likes to nerd out on information and think about them.

My understanding is that mRNA vaccines need a protein to target (since mRNAs make cells produce proteins). What makes SARS-CoV-2 particularly suited for this vaccine type was their spike proteins. My understanding it that the spike proteins are very unique to that virus type, was enveloping the virus and the human biology doesn't have something similar to that, so it was an easy (!) target for this vaccine type.

At least one of those conditions above may not hold true for <insert virus name here>, which can make it difficult/unsuitable for mRNA vaccines to target.

Again, I may be wrong (please correct me and give me the correct information if so), but this is my understanding.