Yes, part of it is a bandwidth issue. The same image shown on every single post can bog things down.
But the other part is I think image signatures really clutter up forums horribly. People create these huge images, often flashing or animated, and they distract from content of the thread, which really should be the focus.
I think the new Garage feature helps with the quick pic of your car. You just click it once to see the thumbnail, but stay on the thread page. But you have the option of clicking again to get more detail on the car.