There doesn't appear to be a technical reason, it appears to just be a default configuration setting that was a holdover from Mozilla's imposed limits when Firefox Send was still around.

https://old.reddit.com/r/selfhosted/comments/1bwqxit/is_ther...

There are other variants with different file size limits https://github.com/timvisee/send-instances Maybe performance based or something depending on those hosting these instances?

Don't forget the encryption step is done locally by the browser before the file is transmitted to the server. As such, you better have a limit.