Why are there so many types of trucks for passenger cars?

Why does each type of passenger car and other passenger equipment (e.g., Metro-North M-8s vs. M-2s, Viewliners, Shoreliners, Amfleet) have its own type of truck? Why aren't there just a few "best" types of trucks?

I can see that a new EMU or DMU would have a higher-tech truck than an older EMU or DMU would have, but why the differences for locomotive-hauled cars?

My question comes from some experiences with rough-riding Metro-North and LIRR M-7s; it seems like since new doesn't necessarily mean better, it could be a good idea to keep older models of trucks and not mess with them for new models of cars.

Thanks.

truck designs are owned by certain manufacturers, they won't let you put their truck put under a other manufacturers train.