The real race begins today in Willow. The reason behind the restart, as best I can tell, is firstly, an attempt to allow more people in on the fun (Anchorage being more populated and easier to get to), and secondly, to create the best trail conditions for the dogs and their mushers. Lack of snow and more roads and driveways crossing the trail have pushed the trail north to Wasilla since 1995, and, for many if not most of those years, to Willow Lake. Here's a bit from Lisa Maloney of Alaska Outdoor Recreation Examiner and the Anchorage Press:
The Iditarod's ceremonial start in downtown Anchorage has become more of a carnival than a racing start, with mushers saving their teams' strength for the real race. Look for higher tension and more game faces at the official Iditarod restart at Willow Lake in Willow, Alaska.So there is still some time to join in on the fun!
The ceremonial start used to be the real deal, but the always-open waters of the Matanuska River meant that teams had to run on the highway bridges if they wanted to run all the way from Anchorage to Nome. Increased highway development means this option is no longer feasible, so instead the teams run as far as Eagle River and are then trucked to Willow for the restart.