Well, public transit projects are often outrageously expensive and take years to complete.
Laval commuters probably pay more in taxes than tourists, so they probably need it more than the tourists.
But, maybe now that the Laval service is done, an airport link will be next.
PS. BTW, going from LGA or JFK to EWR isn't so easy or cheap. The Newark AirTrain only goes to NY Penn Station. From there, you would need to catch bus/subway to get to LGA or JFK. Or, try some of these combinations, all of which require multiple transfers:
http://www.yelp.com/topic/new-york-la-guardia-to-newark
A shuttle van can be reserved, but i've heard that can cost over $30.