Stab in the dark here, Ewan, but mayyybee it's because your character died. Maybe. Just a hunch.
edit: Seriously, why not just answer that question with "Well, no, I haven't been asked. Likely because my character died" other than " Well, JJ Abrams hasn't called me".
Not to mention the fact that he died as an old man, and his character has already appeared as a Jedi ghost before, looking like an old man.
Without Sir Alec Guinness, I think everyone would rather see a new actor than Ewan wearing a ton of makeup and prosthetics trying to look vaguely like him.