Most people don't object to increasing rural highway capacity. However, the increased traffic has to go somewhere, and nobody likes busy streets in their neighborhood. Not in the suburbs, and not in the city. People generally want less traffic in their area, not more.

Urbanists tend to prefer walkable cities. They want to use less above-ground space for traffic and parking, leaving more space for buildings, parks, and public squares. Highway capacity expansions work against that, because they encourage more people to drive into the city.