# Intuition behind interest rate models

I am modelling the 3M yield of US Treasuries using an ARMA/ GARCH approach. Most interest rate models (e.g. Vasicek) describe the process as follows:

$r_{t}-r_{t-1} = some ARMA+ \epsilon_t$

Where $r_t$ is the yield of a bond. Why do you use the yield instead of the return of the yield (relative price change to last period)? For example if you model stock returns you use the return as well. I am very well aware of the relation between price and yield of a bond.