Simply because they don't have the same interest in the results as you do.

Lots of folks are just interested in the ethnicity predictions. Some people are not savvy enough to build a tree online. Others are old school, and did all the footwork at the libraries and city halls and stared at microfiche until they were blind, and they guard their research. Some are newbies and are just starting to research. Some, like me, have research trees that are kept private because they are filled with people I think *might* be related to my dad's birth parents, and I don't want to publish anything until I know it's accurate.

There's as many reasons as there are Ancestry members to not go public with a tree. I've always had very good luck in reaching out to private trees. Put together a pleasant, warm email with what you are trying to learn from their tree, and then wait patiently until they login again and notice that have mail.