Yeah I have always thought about that, its funny he did played lesser succesfull singles in '72 like "Until its Time For You To Go" and even played "For the Good Times" a lot, a song from the same sessions of "Always in My Mind" that wasn?t even released.
On the other hand we must remeber that "Always in My Mind" was released as a B side for "Seperate Ways" and did not chart the Billboard Top 40, its one of those songs that somehow became favourites in time, but passed unnoticed when first released. I personaly think it was big mistake to used it as a B side, it must have been his first A side early April '72, that way it could have been a top 20, and so we may have seen Elvis doing it by the time of the Aloha concerts.
P.S. Maybe the song just bring sad memories to Elvis and he didn?t feel like going through that once again by doing it live.