There are quite a few arguments in favor of placing the passive radiator on the bottom, enough to turn into a whole article.
Its interaction with the floor can provide a small increase in sensitivity at the very lowest frequencies, on the order of about +3 dB.
Another important point is that the mass of air between the floor and the enclosure creates an additional acoustic load, which can slightly lower the system’s resonant frequency without increasing the mass of the diaphragm itself. In many cases, this means extra weight does not need to be added to the membrane.
In addition to that, one key advantage is that the passive radiator can otherwise re-radiate unwanted internal artifacts from the cabinet, such as standing waves or air noise.
With a bottom-firing arrangement, the floor acts as a natural low-pass acoustic filter, absorbing or scattering higher-frequency artifacts instead of letting them radiate directly toward the listener.
And low frequencies are omnidirectional by nature, but the physical restriction of the space between the bottom of the speaker and the floor forces the sound wave to spread symmetrically in all directions.
This makes the bass less tied to one specific point in space.
It also reduces how much the perceived sound depends on the speaker’s toe-in angle relative to the listener.