People buy the best car for their budget that meets their needs best. This is why we have a big estate car, we don't need all that space for the majority of journeys in it, but we do for enough that I have to drive this long thing I hate parking.
The small second car is going this year as we don't need to do enough journeys to justify two cars, neither of us need to drive to /from work now, but we do need a car and some of those journeys will need to be carrying more than the small car can fit, so the bigger, less environmentally friendly one is the one we're keeping.
Tax, insurance, MOT costs etc make it not sensible to run 2 cars when you only need 1 and aren't rich. The environmental impact is a concern, but saving the tax and insurance on the 2nd car is winning with us.
This is selfish of us. The best option for the environment and town traffic would be to keep both, use the small car for the supermarket run and taking the kids to things etc, and only drive the bigger car when we really can't use the smaller, more efficient car. But I can't justify the cost to be slightly more moral.