Tobar said:

Yes but he's not saying "it doesn't matter." That's an entirely different sentence than "It no longer has any meaning for me." Now read these examples out loud and tell me which sounds better:

"It no longer has any meaning for me. Now I know the power of the dark side."

"That no longer has any meaning for me. Now I know the power of the da irk side."

reading it outloud, "it" makes more sense and sounds better. it sounds far more cold and chilling than "that" or "she" or anything which implies he's actually giving thought to it. "that" is far less dismissive than "it"

"she no longer has any meaning for me" sounds like some boy-girl drama...darth vader doesn't process this now.

"that no longer has any meaning for me" still sounds considered and soft.

"it" is the epitome of short, uninterested, mechanical.

from a theatrical/artistic/emotional pov, the mood and atmosphere of the moment is best carried with "it"

the other options soften it and pull me out of the chill