BART in general was designed by aerospace engineers, because people in mid-century America thought that rail was an old outdated thing to replaced with monorails, personal rapid transit and maglev. The people who designed BART from the ground up on first principals (and ignoring everything that rail had done previously.)

The problem with this is that newer technologies were not competing with rail purely on the technological merits, but with rail's massive amount of competition in suppliers and economies of scale. It turns out that with not a lot of effort, you can apply much the same savings and technological improvements to traditional rail, except without the huge added expense of converting to a standard you are currently incompatible with. Newer technologies have the reverse problem, in that they have very few, sometimes even a single supplier, and are basically custom projects with all the expense that entails. So the monorails, PRT, maglev, and other weird system are mostly unique specimens, or very few in number.