Itzli's ability to commune with the dead is so strong because he never stops it's practice.

He developed the ability early on and is original purpose was to seek wisdom from those who've passed on and lived long enough to have acquired it.

He also secretly uses it to talk to old friends.

This is not to be confused with his ability to astral project though. That is something different entirely.
He accidentally discovered that ability while sleeping one night.
Xoc had to find him and return him to his body.

This kept happening until eventually Xoc taught him how to better control it.

It is unknown how she acquired the ability herself....