Because generations of bike parts are not backward compatible, the frame is the cheap part, and old parts become unobtainable,. Everything wears out. I can't run disc brakes on my machine because the frame and fork lack the "braze-ons". I can run a drum, but one can't do that on a stock modern frame. No braze-on. BBs and cranksets. FDs and RDs, their attachments and cable pulls. Rear hubs and their compatibility issues. The bike needs to be designed from the start to accept particular components, whatever they are. Singles are much less complicated, but similarly you can't run even a 9-speed rig on a 70s road bike, the bars aren't designed for brifters, and you can't even get decent tires for those rims.
I was writing for the OP particularly. C'dales were stiff, inexpensive tandems, but the ride and handling were seldom loved.
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