Here's why:

* Simplicity: Hydrogen is the simplest atom, with only one proton and one electron. This allowed Bohr to make simplifying assumptions and focus on the fundamental interactions between the nucleus and the electron.

* Spectral Lines: Bohr's model successfully explained the observed spectral lines of hydrogen, which were a major puzzle at the time. His model predicted that electrons could only occupy specific energy levels, and transitions between these levels would produce the observed spectral lines.

* Mathematical Framework: Bohr's model used classical physics and the quantum theory of Planck to develop a mathematical framework that could accurately predict the energy levels of the hydrogen atom.

While Bohr's model was a groundbreaking success for hydrogen, it had limitations when applied to more complex atoms. Later developments in quantum mechanics led to more accurate and comprehensive models of atomic structure.