Keep in mind this was designed in 2002 using technology that was considered rock-solid & mature at that time. Spacecraft technology always seems outdated because of the lead times required: on something like New Horizons, it's between 15 and 20 years from component design to being used for its primary mission.
In addition, as another commenter noted, the reference oscillator is used for REX (the radiometry experiment on board New Horizons) and quality of gathered measurements for this experiment was one of the primary needs here: being able to use the component for the communications subsystem is just good power/mass engineering at work.
In addition, as another commenter noted, the reference oscillator is used for REX (the radiometry experiment on board New Horizons) and quality of gathered measurements for this experiment was one of the primary needs here: being able to use the component for the communications subsystem is just good power/mass engineering at work.