In order to get houses in the HHA Showcase, you have to Streetpass other people who also play New Leaf. If your 3DS (or 2DS) and their 3DS come within a certain proximity of each other while they are on and in sleep mode, they will Streetpass. I can't remember if Wifi has to be turned on as well, but I think it does.
It's really difficult to get Streetpasses now since the 3DS and New Leaf are no longer heavily played. I got my Streetpass badge by streetpassing my husband's 3DS and my second 3DS over and over. So I only have like 2 houses in my showcase and they're ours.
I've heard about Spotpass as a way of getting things too, but I don't really know how that works because I think it was already a dead feature when I started playing 2 years ago.