From my personal experience places I go will either have lots of ticks or none. The best explanatory variable I can come up with is the deer to human ratio. In places with a lot of deer but few humans the ticks are all just waiting around to bite people that come by. But I'm guessing in places with lots of people, other people have already come by and "used up the tick supply".
Climate plays a big role too. Ticks have a pretty narrow band where they are active. We have ticks in my area for a couple of weeks in the damp, cool spring. After that it's too dry and hot.
