Usually you can only catch them in the morning hours and even then, they only come out every so often. It really comes down to a few things:
-Entertainment - Between Move It Shake It, 3pm Parade, Trolley show, Daper Dans, Citizens of Main Street, and much more, there is so much going on that it is hard to bring them out. Also don't forget the balloon vendors and Photopass photographers.
-Crowds - Traffic on Main Street has increased since the park opened, but we are also seeing more wheelchairs, ECVs, strollers, and such. Those folks sometimes have a hard time just getting down Main Street themselves, imagine a large vehicle. It also depends on the time of year. For instance, during the busy holiday season the parks actually remove many benches and sometimes even props to free up areas where people can walk. Driving a vehicles during a busy day is much harder than during the off-season.
-Nightime - Obviously, the vehicles can't make the rounds at night because traffic on Main Street increases significantly with the nighttime parades and fireworks. It is also a safey concern as it can be harder to see at night.
-Attractions/Training - The Main Street Vehicles are part of Main Street Operations and are sometimes used by entertainment. Main Street Ops has a large portion of its staffing for the parades so those CMs work when those take place and are usually busy with those. The vehicles are also manual (not automatic) so in today's society, there are less people who actually know how to drive manual. Either way, extra training is involved and new hires usually have to wait a certain period before they can train to drive them. Keep in mind that a large portion of the Main Street Ops is also College Program participants who wont' be here long enough to train on them. Finally, if there is time and staffing to drive them, they are usually only out for a loop or two so many folks miss seeing them while they are in the other areas of the park. As for the horses, they are workers too and only spend mornings in the car barn on Main Street. They return to Fort Wilderness for the afternoon.
So, those are my thoughts on the situation. The time window is small, the time of year and crowds play a factor, staffing/training is another, and finally if they are out, you may miss them because it may only be for 7 minutes out of the entire day.