Banter
The developers are usually very slow with this kind of stuff....
I think it is more the amount of time required to crunch the data to make the tracks look and feel right during a race, than developers dragging their feet intentionally. Also, I would believe, it would be a matter of priorities, where new tracks may not be as big a priority as bug fixing.
Other than that, I agree with putting new tracks up. Seeing that variety is the spice of life.
I have already suggested that Imola be added, as a San Marino race.