"Legal and licensing issues" is the official answer.
BG1+2 were on sale during the last Christmas sale and again a few months ago during an Interplay promo, so... my guess is that there's some sort of licensing issue with BGEE. Basically, BGEE just came out and is still buried underneath an ocean of bugs, and the powers that be don't want plain old Baldur's Gate to go on sale and cut into their profits. If it was 50% off like most of the catalog, it would be a quarter the price of BGEE and would indubitably be the better purchase. Thus, no sale price. It's a shame, but them's the breaks.