Honestly, its more than just a lack of marketing, graphics, etc.
First, theres a been a bit of an economic slowdown recently. Now obviously, when people are legitimately strapped for cash, AH is going to (or logically should) be among the first to go, along with Xbox live, WoW, and other non-essential recurring charges. In addition to current players leaving, this also means fewer will join up. The entire gaming industry has felt the effects as well.
Second, the target audience isn't all that big to begin with. Add in the fact that fewer people seem to care even the slightest about history, and smart phones are doing more with gaming than before, and that means that out of the target audience, fewer have sufficient interest to download the game.