Who do you think biological classification keys always present two, rather than some other number, of choices at each step

The biological classification keys always present only two choices at each step because; it is simple, more efficient way of separating organism, without confusion.  

Biological classification is the process in which scientists group living organisms based on their similarities and differences. Originally similarity was determined by examining the physical characteristics of an organism but modern classification uses a variety of techniques including genetic analysis.

Explanation:

if the type of the food available changes, then the frequency of beak types will change, because birds with beaks more suited to the available food will be more successful over time.