They probably just figured people were familiar with his appearance in the films and didn't go too far back.
I've never been quite clear on the 'timeline' of the film series, given that they seem to span (not counting X-Men: First Class) from the early 1970s to 'the not too distant future' which is sorta vague. I'm not sure the makers are either, because I can't actually imagine that James McAvoy would go from how he looked in 1962 to looking like Stewart in roughly 1979 in 'X-Men Origins: Wolverine' (assuming the Three Mile Island connection dates the film to roughly that period).
It's pretty fudged.
That aside, I still didn't find the CGI in Tron: Legacy great either.
'If the Apocalypse starts, beep me!' - Buffy Summers
'The sky's the limit.....' Jean-Luc Picard, 'All Good Things'
courtesy of Saquist