Slowtwitch wrote up a great report on the matter HERE.
Basically the popularity and finisher rate of IMSG was diminishing since its inception, and the powers that be @ WTC thought best to shorten the race to a 70.3 to make it more appealing (and most likely more profitable).
I am sure some folks think this is disgusting, or wimpy (and I totally agree with you if you are in that boat!) but as for someone who lives in the Northeast and just doesn't see early May as a viable time to race a full Ironman, I am welcoming the change of this race with open arms.
While it is possible to train for an early May Ironman while living in the Northeast, it won't be fun, and that is one of the reasons why I do triathlon and Ironman, because its fun. This is also why my A+++ race is usually in August.
In fact, I was looking for a Western 70.3 race to do for this early season, but there is a huge dearth of them currently, other than California 70.3 or Superfrog (are there others?), but San Diego isn't a venue I want to travel to.
St. George, Utah? Lemme explain visually:
Screw racing, I just want to VISIT this area. Bryce Canyon? Zion National Park? Grand Canyon? Just to name a few...
I was chatting with a coworker born and raised in Utah and I said, "I could spend months out there in that area." His response? "Try a lifetime."
So already the screws are turning for 2013, even though I said, "NO MORE RACE PLANS TILL AFTER MONT TREMBLANT!"
Well, I already broke that when I said I wanted to kill the 5K, 10K, and Half Marathon again this Fall.
Why not an early season 70.3 out west and turn it into a race-cation?