Why was the clutch so heavy? Did it serve some purpose or was it just due to the limitations of the technology at the time?

I have certainly driven cars with lighter and heavier clutches (I live in EU, automatics weren't popular until recently and are still far from ubiquitous) but I couldn't tell you why every model just doesn't get a light clutch for comfort. A diesel Subaru I drove had a particularly heavy clutch as I recall, so at stop lights I would pop into neutral instead of holding the clutch down for an extended period.

To deliver very high torque, the clutch plates needs to be pressed very hard together to generate enough friction. This also means that it take a lot of force to pull them apart, if you use a simple lever, as older machines do.

Modern machines may use complex mechanical linkages to make the clutch easy to pull apart but still maintain a firm contact, but that also means higher cost and fragility. Or they use pneumatics or hydraulics to assist, sorta like power steering.