I'm just guessing, but maybe because it's shorter so it gets more widely read than Asimov's other stuff? I haven't read "Bicentennial Man"; will check that out.
BTW I have an Asimov collection I haven't read here yet called The Best of Isaac Asmiov where he lists his favorites of his own stories--here's the contents:
1939: Marooned Off Vesta
1941: Nightfall
1951: C-Chute
1952: The Martian Way
1952: The Deep
1954: The Fun They Had
1956: The Last Question (I've read this one elsewhere)
1956: The Dead Past
1956: The Dying Night
1959: Anniversary
1967: The Billiard Ball
1972: Mirror Image
Here's Asimov's own comments on "Nightfall" there:
"Yet within less than three years of the start of my career it turned out that I had written the best of Asimov. At least, 'Nightfall' has been frequently reprinted, is commonly referred to as a 'clasic. . .One of its advantages is that it has a unique plot. There was nothing resembling it ever published before (as far as I know) and of course, it is now so well known that it can never be published again. . .Yet I was only twenty-one when I wrote it and was still feeling my way. It isn't my favorite. Later on, I'll tell you what my favorite is and you can judge for yourself."
I saw the movie The Bicentennial Man, starring Robin Williams, and enjoyed it. I have no idea if it was close to Asimov's story, since I never read the original story. But, I thought the movie was thought-provoking, and raised some interesting questions concerning what qualifies as "life".