It would have been a Whatcom County service rather then a State ferry. Why couldn't the state provide the service.
Interesting that someone thought about this though.
The state ferry system is essentially an extension of the state highway system. In other words, the state ferries are connecting the same places that the state would connect via highways.
The ferry systems that Whatcom and Pierce Counties operate are more like extensions of the county road system. They are local ferries to places within that locality. They (for the most part) only benefit the local residents. The function of a Whatcom County ferry from Point Roberts to Blaine (or Bellingham) would be essentially for the benefit of county residents. It would make the lives of those county residents better. Or most of the 1314 residents, according to the 2010 census.
Also, I remember a discussion here at one point that much of the traffic over the border at Point Roberts is Canadians crossing -- either to visit their US property in "suburban" Vancouver or to cross the border easily to shop in the US stores there. A ferry doesn't help there.
See the gerbils walk on the ferry in Edmonds! Bree's daily photo blog is at cascadiadaily.com/