Probably the same reason people buy cardboard boxes for cabinets just so they can afford the granite?

I have friends who have concrete counters in their kitchen. They love them. While I'd never trade for my soapstone, I have no trouble understanding. They are goodlooking, practical, custom cast for their kitchen, in the color they chose, and I'm guessing, probably not much different in price as many other popular options. It's a beautiful new home.

not sure concrete sinks are 'so popular', however they are different & can be quite stunning when finished properly

It has to do with "what is a new-ish style". Styles have to change or we would all live in the same style home. Concrete has been on the radar for quite a few years on the high end custom home, all styles travel through the markets. Make sure you understand the maintenance requirements if you are planning on a concrete sink. Concrete sinks are defiantly a more maintainable in bathrooms than kitchens.

One can achieve the 'concrete' look with solid counter surfaces.

well, I've never seen them used in the bathroom so I can't answer that...