Floor..........the only thing under the designers control. He has no way of knowing where any other (wall?) will be located in relation to the driver.
think upside down. the tweeter is more directional so it needs to go in the prime location. I suspect designers shoot for seated ear level. toss in a mid if you have it next and that leaves the space below for bass. Don't forget that most designs have volume inside the cabinet for base drivers so too much too close together may be an issue for designers. Then I bet they don't want the base driver too close to a boundary like the floor so they move away from the floor within the allocated base volume and viola you get knee height.
There is also directional,and audible information coming though a lot of woofers in a speaker system. Some that have sub-woofers in them are mounted on the side of the cabinet,most likely because they have lower crossover frequencies that you may not hear if they were directed at you.My guess is,there is audible music coming from these woofers.That would be another reason for this elevated mounting.
The midrange driver is placed very close to the tweeter to prevent uneven frequency response with peaks and dips due to phase problems close the crossover frequency, where the frequencies reproduced by the drivers have some overlap.
Tweeter at ear height, mid right next to it. If the crossover frequency is low enough, then the bass driver can be placed farther away, such as on 3-way systems with the woofer on the side, back, or on the bottom.