[aledue]

1. The chance of getting popular support for a protest like this diminishes with the number of people you piss off. You may be able to earn some sympathy by targeting SUVs only, but doing the same with all cars wouldn't be a good strategy.

2. There are good reasons to own a car, and in some places a significant number of people may be justified in using one. Blanket targeting everyone would be unfair to those who really need it. By contrast, SUVs don't have any (significant?) utility over a regular car (other than ego boosting maybe...).