Their MacOS client is amazing, so I doubt it's a technical constraint. My guess is that because they don't have size limits on their backups, if all of their customers are backing up terabytes, Backblaze bleeds money, so they depend on most people only backing up a couple hundred gigabytes. Linux users may have a wildly higher amount of stuff to back up such that it's not profitable. And changing their branding to have mostly unlimited except for some people is probably not too appealing either.