it is inert ...
Nitrogen is also inert, and much easier to contain.
Helium is the leakiest gas known to Man.
It has long been used to pressurize liquid propellant tanks, though ... Helium is much lighter than nitrogen, for the same pressurization capability.
Not relevant to Starliner, but Helium also has a much lower boiling point than oxygen and hydrogen, so its use with cryogenic propellants wont cause it to liquify.
Helium is an inert and near perfect gas allowing for more efficiency in pressurized storage systems.