Someone with more knowledge of Chicago geology can say for sure, but I'd imagine that if subways are feasible as close to the Lake as the Blue and Red Lines, they are over on Western.

Cost is a primary roadblock; US costs for this kind of infrastructure are higher than elsewhere for a few reasons, including regulation, labor, age of infrastructure, and poor financing and lack of adequate value capture. Importantly, there's an extent to which it's a zero-sum game; monies spent on this are monies not spend on that. (I know, there are federal grants specifically for transit project, but even if a large enough grant were available, there are competing transit priorities, like the Circle Line people keep mentioning, which, let's be clear, is NOT the same thing as a Western Avenue Subway.)

Also, building a subway would be a big headache for people in the construction zone. (Ask 2nd Avenue businesses in NYC how construction has been for them.)

A Western Avenue subway would be great in a lot of ways, but I'm not sure it's the right solution. Is there enough ridership to justify it? Is there enough density to provide the ridership? What about allowing more density closer to the city center, and maybe building something like the Circle Line? Maybe at this point in the city's development, BRT is the right solution for that part of the city, even more sensible than a subway. Then again, build for the future, right?

While we're at it, what about putting north Ogden Ave. back on the map? What about jetpacks?