Age difference. A child born the day that Elvis died would be 31 today. John Lennon was shot when they were three years old.
For them, "The King" is a golden oldie, not pop music. They can't imagine that he was ever controversial, or even relevant.
His prime was during my lifetime, and while I respect what he did, his passing makes zero difference in my life. As best I can tell, he reached and passed his peak. (Not that his not-quite-peak stuff wasn't or wouldn't have been good, but it could not match the effect of Thriller)
His death is sad, as are most deaths, but hardly earth-shattering, and certainly not "once in a lifetime", given that people such Elvis, MLK, and JFK died during my lifetime as well.
Maybe it's our age difference.