Could is no longer used as the simple past of can in contexts like these, because the tentative or counterfactual use of could is now the “default” reading and using it in a factual sense would confuse the reader.

This is another reason why non-modal verb phrases like be able to and be going to and be willing to are replacing full modals; these don't have the ‘dual’ use of their past forms, so it is less ambiguous to say

I left early and was able to get a seat.

Note that this sentence is entirely acceptable:

I left early so I could get a seat.

There the could is non-factual—it is equivalent to would be able to or might be able to.