Why do we use past perfect instead of past tense here?

In general, both (a) and (b) are possible.

a. We couldn't put down the floor until the plumber finished.

b. We couldn't put down the floor until the plumber had finished.

If you started putting down the floor right after the plumber was done, I think either (a) or (b) works. But (a) is simpler, so you might want to go with (a).

If you didn't start putting down the floor right after the plumber was done, but you waited some time in between, I think only (b) works.

Since we do the flooring and the plumber does the plumbing, the second scenario is more likely to fit the sentence. In that case, (b) is better, despite the more complex form.