Translation:That house had few windows, and so it was not very bright.
We have no reports in this exercise from the past few weeks. That is why I wrote that advice again. Not sure if some reports are getting lost somehow?
I took your sentence, copy-pasted it into the debug field in the system and it gave me the green light meaning "this is an accepted answer" - then I also manually checked the various possible options and saw that, indeed, this sentence with "that is why" is one of the accepted ones. And the exercise had last been modified 2 years ago (no new answers added since). I only modified it afterwards to add "that is why" into a couple of other word orders.