A can on the floor would be super impractical.

There needs to be one in every stall (imagine having to find somewhere to set down a soaked feminine product so you can finish up, and then leave the stall with it in your hand 🙃 a gendered bathroom is one thing but either way I imagine you'd have way more folks just opting to flush. Best case scenario, it gets wrapped in a wad of toilet paper that also has to be supplied and attended to.)

There's only so many places on a stall floor to put a trash can. Next to the toilet is out of sight enough that you're gonna have more flushed tampons. Next to the door is in the way enough to get kicked and knocked over. In a larger stall it could easily end up out of reach.

You'd also be asking a janitor to move a trash can out of the way and replace it in every stall every time they mop, on top of bending down to clean and change each one.

On that note, anything that gets on the floor will also get on the trash can, meaning any metal will probably be exposed to corrosives (urine, cleaning chemicals, even standing water).

Even in terms of cost; the ones on the wall can be as cheap as cutting, bending, and installing a piece of sheet metal, and will basically last the lifetime of the stall.

I honestly can't think of a single good reason for it to be anywhere else.