Elvis was at his best during his early wild days, then was drafted into the US Army. John Lennon said "the Army finished Elvis" but I don't think it's as simple as that. My guess is that his manager Col. Tom Parker sat down to have a fatherly talk with him after he got out and said something like "you've got to clean up your act boy and appeal to a wider audience by doing some soppy sentimental songs and being nice and polite, then you won't upset the establishment and they'll put you in movies and give you regular concert bookings and you'll make a million!"
And that's how it went. In my opinion Elvis was finished as a rock 'n roller by 1960 and everything he did after that was just sugary-sweet stuff without any fizz.