Early, low-hanging fruit?

That, and larger plants. Things like concrete containment vessels have costs relative to the surface area of a hemisphere, while power output scales with the volume. But the big driver was economies of scale. Building multiple copies of the same or similar design means you can have longer production runs of steam generators, pressure vessels, turbines, etc.

I.e., cost has risen monotonically since the 1950s.

Incorrect, it fell from the 1950s through the 1960s.

I.e., cost has risen monotonically since the '60s. Costs were all over the map in the 50s.

In the early days, vendors didn't know how much NPPs would cost, so they priced them to sell, so they could gain that data. Later prices reflected that experience.

One can view the increase over time of nuclear costs as the replacement of wishful thinking with actual experience.