Northern outer suburbs of Boston - the Bermuda rectangle of cycling

I was browsing the Strava heat map and this black hole caught my attention:


Here it is a bit zoomed in and compared to Google maps at approximately the same zoom level:


What's up with that? Why are Billerica, Burlington, Wilmington, and Tewksbury so relatively unpopular with cyclists? There must be some cyclists at least originating their trips there...