Cost and precedent. It's inexpensive to get a prefab kit that only needs to be bolted to a couple spots on the floor and wall, less finicky than building a partition all the way to the floor.
Also USians are used to it, so we don't complain :)
You can get partitions that go all the way down, they just cost more.
But the reason for the shorter partitions isn't generally cost.
1. Security: you can see if someone is still in the toilet when it is closing time, etc.
2. Cleaning: it is easier to clean with a gap, more clearance for the mop.
What I really wish we would get is stall door locks that indicate if locked or not. A properly hung stall door should naturally move to the open position but most are not properly hung. So you have a row of stalls all with the door closed and no idea if they are occupied or not. Which means you are either pushing on the doors, looking under or through the gap. It's Barbaric.
Edit: OK found the real reason. ADA requires 9" toe gap under partitions for maneuvering. 12" gap for compartments for childrens' use. If the compartment is wide and deep enough those requirements go away.
ADA Standards: 604.8.1.4 Toe Clearance
Those reasons are valid, but I'm not sure they beat "my vendor has a kit for this."
Like, even if you wanted a partition that went to the floor, a lot of vendors don't even have them. That's why I say precedent is a big reason why.
Ah yes, of course its regulations. I'm not sure how "valid" those reasons are, the amount of people dying in bathroom stalls over here is not getting out of hand last time I checked (which is never).
>>What I really wish we would get is stall door locks that indicate if locked or not.
Wait......that's not standard in the US????? I've never seen a bathroom with a door lock that doesn't show its state on the outside, even if it's a simple mechanical latch.
Also USians are used to it, so we don't complain :)