Question

Why was automatic garbage collection so slow to be adopted by imperative programming languages?

Answer 1

In general,  it's because because it's more complicated to implement
Since the garbe collection is fairly new, it's harder to bolt on a garbage collector to an existing language because it may cause conflicts that's hard to identified.
Becuase of this, it is much more easier to build a new language rather than integrating it with the old one.