It may also be that when we have several choices, some will be viewed as a poor compromise. A 24" sits between a 20" and a 26" so it is too big to be a small wheel, and if i want a bigger wheel, might as well go for a 26". Generally a 24" is chosen for a smaller rider that wants the advantages of a larger wheel but won't fit something with 26"

The same thing happens with the 650 size for road bikes. It is usually used for smaller riders so that the frame can be made with adequate proportions for a smaller rider. Most roadies will pick a 700 if they can fit it.

There are however several advantages to smaller wheels: lower the center of gravity, reduced rotational inertia, more responsive handling...... I am a big fan of minivelos for those reasons, but all of mine wear 20" because of tire availability and choice.