Piggybacking on Mike's point, with them airing the $25/100K versions, I'm thinking one of two things:
a) they felt it would look cheap to brand the new version with a dollar amount, when they're airing 25-year-old episodes offering a top prize of $100,000, or...
b) they don't want to confuse viewers who are wondering why there's two episodes of "The $25,000 Pyramid" or "The $100,000 Pyramid", esp. if they air close to the same time of day, i.e. 1982 $25K at 6, 1985 $100K at 6:30, 2012 $25/100K at 7.
Of course, that could all be fixed by calling it "The New $XX,000 Pyramid", but maybe they don't even wanna confuse themselves. Hell, I'm confusing myself! :-P