Its just too wet there
Generally, fires don't happen where it is wet. Here is a map of the fire regime in Pre-Columbian America:
Northern New England gets practically no fires, with a fire frequency of every 200 years or longer. Southern New England sees more frequent fires, maybe ever 40 years, but those fires are typically understory only.
For a region to be fire resistant, it must meet two criteria: it must be 'humid' and it must be 'humid' year-round. New England meets both of these criteria. Here is a map of precipitation versus evapotranspiration for the US:
Note the areas in green have more water falling from the sky than returning to it via evaporation and transpiration. New England is one of the wettest parts of the nation by this metric. There is plentiful rain/snow, generally heavy cloud cover, and relatively low temperatures to reduce evaporation.
The last piece is being wet all year. By looking at Boston's climate data, we can see that Boston's precipitation is evenly distributed throughout the year.