Nail salons have been opening right next to each other in my area, so I've been pondering this for awhile. It may just be a question of starting capital. You don't need much to start a nail salon. They don't have special furniture (the foot places do, but I'm just talking about the manicure places). They stock only a tiny amount of supplies. They don't need a web presence, or a lot of marketing in general. They don't require special equipment or stock nontrivial amounts of dangerous chemicals.

The biggest cost is labor, but labor money isn't something you need up front, since presumably at that point you have money coming in.

I realize everyone who's watched Breaking Bad thinks it's all about money laundering, but that's not as easy to do anymore, at least where I live, and the nail salons are proliferating.