Great question. Mesonets are actually hard to design and implement. There are many moving parts and startup funds are often difficult to come by. In Maryland we benefit from having a science minded electorate and state legislature, as well as small number of counties. This results in a project getting off the ground quickly.
As for other networks, Wisconsin is building out their mesonet pretty quickly, and Pennsylvania has their first 8 sites going in the ground this summer. I've heard several rumors that Maine is starting one up, but I can't find any official source on that.