I tried to find some statistics on the 'net of how many Ironman Finishers there have been per year and to compare this against the trend of Marathon Finishers. It's a totally subjective and unscientific thing to say, of course, but it is my impression that more and more people are turning to the Ironman as the "ultimate challenge". There are always harder things you can do: there is even a 20 times Ironman distance event where you do all the swimming in one go (in a swimming pool - imagine having to do 3,040 lengths!), all the cycling and then all the running - you have a certain number of days to complete the whole thing. But the Ironman has captured the public imagination - especially in America - in much the same way as the Marathon has.
The reason is History. Why is it that the most coveted running distance is the seemingly arbitrary 42,195 meters? There are many articles on the internet that explain how we ended up with that distance but my point is, were it not for Pheidippides and all the other quirks of history that lead us to 42,195 (or 26 miles and 385 yards), the Marathon would not have the same prestige or nostalgia. Imagine if in the 70s, someone had decided to round the distance to 40km? It wouldn't be the same. Nowadays, most people know what it means to run a Marathon, have heard about the "wall" and may even know that breaking 3 hours is a very good result.
Much the same happens with the Ironman. The Ironman came into being as the result of a debate Naval Officer John Collins had with some colleagues over "Who was the fittest - the best swimmer, the best cyclist or the best runner?". To settle the question, they decided to take the three toughest tests the island had to offer and came up with a grueling Triathlon: 3.8km swimming, 180km on the bike, followed by the Marathon. Thus the Hawaii Ironman (now Ironman World Championship) was born in 1978. The name came from John Collins, who proclaimed that the first person to cross the line would be called the Iron man. On the last page of the race rules was the slogan "Swim 2.4 miles! Bike 112 miles! Run 26.2 miles! Brag for the rest of your life" which is now a registered trademark! By the way, the 17 hours cut off time is the time that John Collins himself took to complete the even in 1978.
So the Ironman meets the requirements of having history but it was probably the epic battles between Dave Scott and Mark Allen that really raised it's popularity, again, especially in America.
In 1989, Mark Allen finally reversed Dave Scott's 6 Ironman World Championship wins, by swimming, cycling and running neck and neck with him until, finally, the one with more gas left in the tank won. Mark Allen went on to win the event a further 5 times, equaling Dave Scott's record.
Funnily enough, arguably the other person to have had the biggest impact on raising the Ironman's profile was Mark Allen's wife, Julie Moss. In 1982, she was within a couple of miles of crossing the finish line as the first woman when she collapsed, probably from dehydration. She ended up crawling (literally) to the finish line and was pipped by Kathleen McCartney at the post. Millions of people saw her epic struggle on television.
Currently there are 25 official Ironman events around the world and 11 of these in America. Lately, though, the Ironman craze has been growing in Europe and the latest event to be added to the calendar is in Wales. There are also many so-called "Iron Distance Triathlons" which are simply not part of the WTC (World Triathlon Corporation) franchise. They very sensibly keep a very tight control over the Ironman brand and the quality of the events they run.