Because CP-violation is actually the only detectable way to discern between right handedness and left handedness. That is, without CPv, we would have no way to know that we aren't in a hypothetical mirror universe with all rotations reversed. Every other physical interaction behaves identically left or right.
So we have a few options to deal with our rotationally-challenged alien friends:
1. Hope they can parse far down enough into the dictionary to understand what CP is and either know it or can test it.
2. Communicate using a shared reference, like pointing out two quasars that rotate relative to each other. (Quasars are pretty good galactic reference points.)
3. As long as it doesn't affect giving directions, just don't worry about it, the physics works out the same. If we ever meet and they try to shake our left hand, well, they get their very own "oops I guess electrons are negative then" situation. "And that, kids, is why you always negate earth-radians before using them in a formula."
Veritasium makes a great video about symmetries that's very relevant here [2], and might suggest a slightly more practical way to correctly communicate handedness to aliens. The full CP violation experiment isn't necessary, as long as we assume that the aliens live in a universe made of mostly matter (as opposed to antimatter) like ours, and we can communicate the parity violation experiment (~measuring the preferential atomic decay direction of cobalt atoms near 0K in a magnetic field [3]), they should be able to reconstruct an absolute reference for rotation.
[1]: https://en.wikipedia.org/wiki/L-Glucose
[2]: https://www.youtube.com/watch?v=yArprk0q9eE
[3]: http://www.physics.utah.edu/~belz/phys5110/PhysRev.104.254.p...
