User blog comment:Arcadebomber/Need help!/@comment-4674838-20120821024503

The problem with that room is that it doesn't meet the NPC block requirement for houses.

At night, NPCs AI requires them to stand on at least 1 solid block (NOT a wood platform) and that block must be next to a table/chair/etc. Basically if you replace those wood platforms in the floor with solid blocks, you will meet this requirement. Or you can move the table/chair around a bit.

By the way, you are seeing a bug. In rare cases, it can say "This housing is suitable." when it actually isn't. That is a bug in the game, often having to do with the NPC block requirement or the Corruption requirement for housing.