The ATK component will be pretty much unchanged from the Ares I solid booster it had under development for NASA and the Ariane 5 component has been tried and tested dozens of times in satellite launches for the European market already.
In short, by stacking the two rockets, launch costs can be lowered, tech developed for the Constellation Program needn't go to waste and the upper stage is already in operation.
Also, the ATK/Astrium partnership comes at the perfect time for President Obama's new vision for NASA spaceflight: stimulating commercial space interests. $200 million in NASA seed money (the Commercial Crew Development-2 competition) is up for grabs and there's no shortage of private firms competing for a share of the prize.
"The Liberty initiative provides tremendous value because it builds on European Ariane 5 launcher heritage, while allowing NASA to leverage the mature first stage," said Charlie Precourt, Vice President and General Manager of ATK Space Launch Systems, in Tuesday's press release. "We will provide unmatched payload performance at a fraction of the cost, and we will launch it from the Kennedy Space Center using facilities that have already been built. This approach allows NASA to utilize the investments that have already been made in our nation's ground infrastructure and propulsion systems for the Space Exploration Program."