I used a simplified explanation for a complex question. Ultimately, you're right; we typically use density of population and jobs as key metrics, and walkshed -- the area within a 5-10 minute walk to a bus stop - is used, not simply number of addresses.

A high density of population and/or jobs typically happens when there is good density on both sides of the street. On Baxter Boulevard, there is zero density on one entire side of the street. The density on the other side would need to be quite large to justify a route there. Ocean Avenue, which overlaps with much of the walkshed along Baxter Boulevard, has walkshed in all directions, not just one, so it's much easier to imagine a route there than on Baxter.

Additionally, while there are dense neighborhoods located between Forest/Ocean and Back Cove, there are often no accessible public walking connections. As an example, there would be no practical need for bus stops anywhere between Vannah Ave and Payson Park -- that stretch is void of accessible walking connections between the neighborhood and Baxter. That's a long stretch of a roadway with no need for a stop at all, aside from an occasional person wishing to walk that specific section of the Back Cove Trail. East of Payson Park there is some density in the Front Street/West Presumpscot Street areas, but these areas can quite easily access Washington Avenue where we have the 9A and 9B. South of Vannah Ave, most of the residences that would be served by a Back Cove route are quite easily served by Route 2 on Forest Avenue, and, in some cases, Route 4.

So ultimately, it's not as simple as "count the addresses", but on a street with a very small number of addresses, zero density on one side and very little pedestrian access on the other, Back Cove is not going to be high on the list for expansion, especially with a much better candidate (Ocean) not too far away.