Plus Floppies are more convenient to carry your files with you and then go to computer, or some body else's in another office, and load your files there. Portability.
Floppies are an electromechanical part that is unreliable.
Transfering one 1.4MiB floppy worth of data over 115200 serial is going to take 102 seconds, if my maths is less broken than usual. In 1 minute 42 seconds, the "convenience" of carrying the file via sneakernet (execute copy command, unmount floppy, get to instrument, retrieve disk, move disk, insert disk, mount disk, copy files out of it) is going to be beaten. For trivial things like a file in the 10s of KiB, serial is so much faster it's silly.
Besides, RS-232 is soo archaic today (except where it isn't; I use serial console at work every week, at least) that it carries its own aura de cool with it.
I don't understand the hate for floppies, what else did you want scopes to use back then ? USB ? There was no USB.
Serial link faster ? That's irrelevant, that implies that by some magic wherever you decide / need to put your scope, somehow there will be a computer right there on the bench next to the scope, and that it will have a serial cable and the S/W to go with it... didn't work like that in real life at my school at least. There were no computers in the class room in physics, so the teacher would grab the scope (only had one, was so expensive), bring it to a particular bench in the class, put stuff on the floppy then grab the floppy back to his office, that might a few meters away, or on another floor or even another building altogether.
You need a portable media, and one that's easily usable by the desktop computers of the day. So it was either3.5" floppy or nothing. Simple.
Hell even if by some magic there WAS a computer with a serial cable available right next to the scope, then what ? You transfer your file to that PC, then what ? How do you transfer that file to your computer in your office so you can create your report / Word document with it ? You get it from the network ? There was no network back then in my school, to speak of. So used floppies to transfer files from one computer to another. And let's say that OK, there was ONE computer in the class room... class room is very large, scope might requires a 10 or 20 meter serial cable to get to the computer... you don't get silly fast speeds of R232 at that sort of distance, no you just crawl... quite possibly even much slower than a floppy drive would write the file.
So take the scope to the computer at the other side of the room ? Silly, no practical. Once you have setup your scope / experiment, it stays there until not needed anymore. But writing the file to a floppy and grabbing that and taking it to any place to any computer.. yes, that, is practical.
I don't see, back then, how you could offer the same practically in any other way. Surely all TE manufacturers thought like me that ",5" floppies were the obvious choice, and surely all the customers that paid extra to tick the " FDD " option though that too.