Had a question from a homeowner which kind of caught me flat footed while installing a couple of mini-splits last week. His question was:

"Why do they still use copper tubing for the linesets?"

I have a number of guesses:
chemical compatibility with refrigerants
burst strength due to the high pressures involved (relative to gas & water lines),
ductility/ease of installation
to answer this question, but wanted to hear from others on this.