I was reading Peter Mann's Lagrangian & Hamiltonian Dynamics, and I found this equation (page 115):

$$p_i := \frac{\partial L}{\partial \dot{q}^i}$$

where L is the Lagrangian. I understand this is the definition of conjugate momentum, but I wanted to know if there is a particular reason for the momentum index to be a lower index and the coordinate index to be an upper index. Is it simply the author's preference or there is a deeper reason?