Why does a decimal sized 26" tire fit my rims but a fractional sized 26" tire doesn't?
You ask what to look for when buying MTB rims.

The most obvious thing would be ask for Mountain Bike tires (slick tires in your case) at the shop. Just using the words "mountain bike" is enough for any shop attendant.

Now if you want to take a tire and, just by looking, tell if it fits your rims, you must look for the ETRTO code, which states the inner diameter of the tire bead in milimeters, which is 559 for MTB.

So you have 47-559 (read right now from one of my tires) to be equivalent of 26x47 (47mm wide, or roughly 1.9)

Hope this helps