Short answer: the Town of Belmont is filled with your typical pseudo suburban NIMBYs who will stop at nothing to get everything they want and nothing you want.

Long answer: funding trail improvements takes time. Designing them should be the fastest part, since you literally just need to throw down 10 to 15 feet of asphalt and design a few connectors. Things can get a bit complex if you're crossing multiple roadways, and if multiple Towns/Cities/State Agencies need to be involved, things get more annoying. In this case, Belmont's section actually runs parallel to the existing MBTA Fitchburg Commuter Rail Line. This is due to the Central Massachusetts Railroad (of which these rail trails are being built on the old ROW of) ran parallel to the Fitchburg tracks from the Fitchburg Cutoff (now a path in Cambridge next to Alewife) to the old Clematis Brook Station. This is seen nicely on Wikipedia under the Stations and Junctions section of the Mass Central Railroad page.

Ultimately having to deal with the MBTA almost certainly adds some time to things. Throw in funding and their extremely slow planning process... and yeah, this is taking way longer than necessary. At least there's some quieter side streets to take that roughly follow the existing ROW up to Belmont Center... from there I can't wait for the path to exist to avoid taking the busier streets around there.