You know, for someone who was so unpopular, he certainly sold a heck of a lot of books....
Let us not forget that, for the last several years of his life, nearly every one of Isaac's sf books ended up on bestseller lists, sometimes for fairly lengthy periods. Or the fact that nearly all of his work sold quite respectably, including numbers of his science books, which often went through various editions and numerous printings. Ditto his Guide to the Bible and Guide to Shakespeare. This sort of track record for a man whose name (as either author or editor) is on over 400 book titles, doesn't exactly spell "unpopular" in most spheres of reference.
There is also the fact that, from at least the 1970s, Asimov tended to receive hefty numbers of fan letters a day -- sometimes into the hundreds, apparently. And the the fact that he was frequently asked by the leading journals, magazines, newspapers, and the like, to contribute articles on all sorts of topics. That his name was big enough for a publishing firm to take a chance on selling not one, but
two new magazines based on that name recognition alone. Can anyone name any other contemporary writers to have that sort of recognition and respect?
Asimov may not have been in the "Harry Potter" league as far as sales during a relatively brief period, but for nearly 50 years his work was almost a guarantee of sales. Somehow, that just doesn't sound
anything like unpopularity to me....