Because ACOM is not a one-trick, two-trick, or even three-trick pony. The experience for a genealogist (the fastest-growing hobby worldwide)on ACOM is just better than on other sites. It's faster, it's easier, it's more intuitive.
I had already mentioned that in previous posts. However, it's not just indexing the 1940 census, it's how it's presented online. I've looked at browsable images on both Ancestry and Family Search, and Ancestry's are far superior. Also, for the beginning to middle range of genealogists, Ancestry's way of finding approximately the correct enumeration district is easier, while FamilySearch's requires a bit more knowledge from the searcher.