Uninformed speculation: I would hypothesize the mRNA process could react faster to flu mutations, the current vaccines are kind of trying to guess which strains are going to be important in the next flu season. It is also possible the mRNA technology could cover a broader spectrum of flu antigens.
Influenza genetics are weird: https://www.cdc.gov/flu/about/viruses/change.htm
https://www.nature.com/articles/nature06945
https://www.nejm.org/doi/full/10.1056/nejmp0904819