Fenway/Kenmore had the unfortunate fate of coming on line just as streetcars were making today's inner suburbs an option to those working in the city. The ability to build a house with an actual grassy yard (a revolutionary idea at the time) away from the ills of the modern industrial metropolis proved to be much more attractive than the typical narrow city lot with next to no grass and windows on only two or three sides of your home.
Suddenly, the middle class could now have a little of both the city and country -- once something only the wealthy could afford. The modern suburb was born.
As a result, the majority of housing stock eventually put up in the emerging Fenway-Kenmore neighbordhoods were enormous low- to medium-income tenements (e.g. all of Fenway), while the few areas left for the dying townhome breed (such as Bay State Rd.) developed nowhere near the pace they did for the blocks east of Mass Ave.
Commonwealth Ave. in the BU area was originally to be lined with lavish townhomes just like the established section (the Jewish community even built a temple there in anticipation of a residential neighborhood), but obviously that never panned out, and in their place sprang up
Boston's first "auto mile" along with the university.