I think a lot of it has to do with drums (instruments in general) selling so quickly that companies didn't need to worry about making things beyond the Big Kit (22" bass) and Little Kit (20" bass). They were going to sell them all regardless.
The only real big floors I see early-to-mid 60's are with the Deluxe-o flagship kits like the Ludwig Blue Note. Easier to find 18" basses than floors.
That being said, I once saw an early-60's Slingy gold sparkle 12/12/20 floor/22 bass, with dual rails for the toms, all factory. That was a weird one. But those kind of things are out there. Good luck!