I don't buy that it's "the simplest". Just about every major Linux distro ships w/ SFTP enabled out-of-the-box. How is installing an FTP server easier than just using the built-in SFTP server?
I've been trying to actively discourage the use of FTP for the last 10+ years. It's not an option because it passes passwords in-the-clear. Protocols that pass cleartext authentication should just be off the table today.
I find that positive attitudes towards FTP often seem to correlate with positive feelings towards Telnet, and both seem to correlate with "Not really that comfortable with Nix"*.
The number of times I have had to correct tech-ish friends when they talk about "telnet-ing" into servers is frightening. All of them were relatively technology literate but either didn't do it for a living or got into doing it for a living "by accident". Think your physics major buddy who has only ever used windows on any computer that he owns.
You put people like that in the position to make a call, and I assure you you'll have an FTP server running somewhere in 5 minutes flat.
I believe that random Windows authoring software is vaguely more likely to have FTP built in than SFTP. (But finding good SFTP software for Windows, e.g. WinSCP, is not hard, so this is not much of an argument.)
It's also the case that SFTP requires giving someone an actual user account on your UNIX box, and preferably knowing enough about how to set SSH up to restrict them to SFTP access only. If your server is much more valuable than the data and you don't trust yourself not to get SSH configuration subtly wrong, it's not terribly unreasonable to prefer installing an FTP server to adding a local user and giving someone else a password to it.