Because run is not past tense.
If you're asking why "we played, we ran and we sang" isn't accepted, it probably should be.
But not only is a matter in mistake with "run". Duolingo wants it will be reduce personal forms at the following verbs ( as you see in their translation ). I wrote : "we played, we ran, we sang " and it wasn't accepted too. But I don't treat it as the mistake.