There can be many factors that can be considered while choosing any theme while building your wordpress website like :

1) Speed of loading
2) Free or Paid
3) Future Support
4) Re-usability
5) Options Available
6) User Experience
7) Responsiveness
8) SEO friendly or not
9) Ease of customization
10) Security

These are the major factors and one good developer will consider all these factors while choosing the theme 🙂