The houses on the right are too close to the corruption. Try removing all corrupted stuff for some distance.
If you are in hardmode you can throw hallowed stuff around to turn it into hallow and fight the corruption that way, otherwise you'll have to use purifying powder (that you can buy from the dryad). Or bomb/mine it all away.
Make sure the corruption cannot creep back again after you've cleared it, or the houses will be invalidated again. (For example by digging a four wide tunnel around the part you want to defend, and line the walls of the tunnel with non-corruptible blocks. (for example all sorts of bricks work for this)