Why doesn’t the Red Line run north to IAH?

I visited Chicago last year and it was so convenient to just take the subway from the airport to get downtown. I know our downtown isn’t the same kind of destination but it still makes sense to me that it should be easily accessible from our largest airport.

It seems that the route could have very easily continued up Irvington Blvd where it branches from Fulton St and then ultimately followed Hardy to the airport. That may not be an option now, but I’m sure they could still make it work with the current system. The line just might have a few harsher turns.

Is there a good reason the original design didn’t track up to IAH? Is there a plan to expand in the future?