Simply put... maybe they prefer what is at the bottom?

You haven't mentioned what is at the top middle or bottom, Or how deep you are talking to the bottom.

There is cardboard at the bottom. It's about 8 inches deep of compost

Have to go with my original guess.

to lay eggs? I have read the eggs are more often found at the bottom of a bin.

My first inclination is the moisture content. Is your bin too dry? Worms will go where they are most comfortable. Moisture is always highest at the bottom of plastic bins. Does your bin have a sour smell? You might be overfeeding and conditions are not good near the surface. Does your bin have good ventilation? These are some of the questions you should ask yourself. Listen to your worms.... they are trying to tell you something.