They might but if I recall MRNA stuff is pretty new - and getting the clinical trials through the entire process and approval takes quite a long time.

Looks like doctors were able to treat brain cancer with a mRNA derived vaccine:

https://ufhealth.org/news/2024/uf-developed-mrna-vaccine-tri...

As somebody who unfortunately has a Stage IV diagnosis I have been researching mRNA and there have been promising results such as the MSK pancreatic study below, but still much to be ironed out — they had half the participants get a response but the other half nothing, even though each treatment was individually targeted and customized. They are doing a larger study now to try to see what other factors may be at play.

https://www.mskcc.org/news/can-mrna-vaccines-fight-pancreati...

mRNA vaccines need a target, and if there is a target, there are several approaches that already do this (anti-body drug conjugates), and sometimes work, sometimes doesn't.

I don't think anybody thinks it "didn't work out". It's still actively ongoing:

https://www.mdanderson.org/newsroom/md-anderson-curevac-ente...

https://investors.modernatx.com/news/news-details/2023/Moder...

(from 2021):

https://link.springer.com/article/10.1186/s12943-021-01348-0...

https://link.springer.com/article/10.1186/s12943-021-01348-0

One big trial still going: https://clinicaltrials.gov/study/NCT03739931

They do work, mainly for keeping a remissed cancer at bay.