"Could not" means that there was some reason why the activity was not possible. "Did not" means that either the people "could not", or for some other reason they chose to not undertake the activity.

Can not could not?

In statements, we use "can't" for present tense, and "couldn't" for past tense. ... "Couldn't" is used to refer to impossible things in the past. So, we cannot use them interchangeably. So, present tense, "can't," past tense, "couldn't."

Was not able or could not?

We use could for general ability. However, when you want to say that somebody did something in a specific situation, use was/ were be able to or managed to (not could). For example: There was a boy in the river but people were able to save him.