They're good options too. I use Squarespace for one of my businesses because it needs to be updated by non-web-savvy people and it was the easiest route for that particular business.
Squarespace seems to have really carved out a nice little niche for themselves. They've also really gone all in on podcast advertising. I'd be curious if/why there is some particular audience for podcasts that particularly aligns with Squarespace's product.
I hear their ads constantly on This American Life, Radiolab, Serial....or simply high quality general interest podcasts covering a wide range of subjects. I'm just guessing here, but I imagine the typical audience might be people that are educated, interested, curious, with good taste, somewhat close to the pulse of time, without necessarily falling into hipster territory, ...and not afraid of trying to build a website on their own (I'm guessing that's how they see using Squarespace), whether for their small business or their personal blog for niche interests.
Sure, a very general and possibly very wrong assumption on my part, but I think it makes sense to target those listeners.