Why does Gaia use only calcium NIR lines for stellar radial velocity measurements?
The Ca triplet in the near infrared are extremely strong resonance absorption lines. They are by far the strongest features in the near infrared spectra of cool G,K,M type dwarfs and giants, which will be the majority of the stars observed by the Gaia RVS.
The Ca triplet lines are so strong that even in low metallicity halo stars, that have little Ca in their photospheres, these lines are still strong enough to measure radial velocities.

The lines are much weaker and much broader for hotter O, B and A stars, and measuring radial velocities for these will be difficult and much less precise.

You can have a look at an atlas of the Gaia Ca triplet region for stars of  different spectral types in Figure 2 of Munari et al. (2001). http://cds.cern.ch/record/531022/files/0109057.pdf

I should also add that these three lines are not the only features used to determine the velocities, they are just the strongest features in the spectra of most stars.