Tense – ‘Weren’t Playing’ vs ‘Didn’t Play’

Okay, so this is a very specific answer to a very specific question. In this specific case, where there are sports teams involved, there's actually a difference in how I, as a native speaker of British English, understand those terms.

We weren't playing.

This means that the team wasn't scheduled to play. So, in the case of the weather, you might say:

The weather was very bad, but we weren't playing anyway so it didn't really matter.

On the other hand, we have the meaning you want in the other option:

We didn't play.

That means that you didn't play. Playing was not done by you. For any reason, you didn't play. In certain contexts, it might have been that you were supposed to play but didn't, or it could mean you were never scheduled. Thus we get to your example sentence:

The weather was very bad, so we didn't play.

This means what you want - you were supposed to be playing, but the weather was bad so you didn't.

Now, the fact you have the so in the weren't playing example means that native speakers will understand what you meant. You've indicated a causal link between the two statements, and thus you must mean that you had been intending to play and then didn't. However, it's more idiomatic, in my experience, to use we didn't play in that situation.

I've no idea if this is unique to British English or not.