Flea bites do indeed tend to be more prevalent in areas on the lower body, such as the feet and ankles. This is because the feet and ankles are in close proximity to the ground where fleas reside and hide.
When a potential host walks by, the fleas can detect the body heat as well as vibrations in the ground and air, and they use their incredible leaping ability to launch themselves toward the host.
So, the reason fleas bite the feet and ankles so frequently is because those body locations are within easy jumping distance of a flea that is laying in wait for blood meal.