As someone mentioned before, Indy is a niche market. Like other media of that sort, you only really see a great deal of marketing/products for it when there is an upcoming film. That's just the way it is.
Nobody mentions the real problem, though: the property was essentially abandoned between films. Sure, you had some small things here and there; a couple video games Expanded Universe books, but that was it. And don't get me started on Young Indy.
I know Star Wars also had a large gap between films, but there was always a market for it; things kept getting released to renew interest. I can't honestly say there wasn't a time when I didn't see Star Wars or Transformers action figures in a toy store somewhere. To keep a media property at the forefront of the public eye, you have to either A) Keep releasing quality products at a steady pace, or B) Keep making movies/TV shows (in the same vein as the movies).
With the slipshod quality of the "Kingdom" figures (the exception being the Lego Indy line, which while awesome, is a tad expensive), I can safely say "A" wasn't met, and given there was 19 years between times we saw Ford deck someone in the hat, "B" wasn't satisfied either. Properties like Star Wars, Star Trek, Dr. Who, Transformers, etc. almost always have something new.
To put not too fine a point on it, most of Indy's strengths come from the old films, not what's new, because there isn't anything new being done with it. The smartest thing they could have done was 2-3 more Indy films in the 19-year gap, and they didn't. If that was the case, you'd see Indy being a little more high-profile than his current "niche" status.
