They didn't go straight to Steamworks because they didn't have the expertise in PC development at the time.
The game was ported to PC at the request of the fans, FROM was very vocal about their lack of PC experience and made it abundantly clear that it would be a direct console port from the xbox version (which means xbox netcode API, which is GFWL).
They might see it as good business to port it over to steamworks now that Namco has given them a bigger budget for DaS2 and with it a bigger development team. DaS2 is being marketed as a AAA title, and with that should come a larger audience, which often prompts people to go and play a developers back catalog if they like the game enough. If DaS1 no longer works than that's money not in their pocket.
As it was when they originally ported it GFWL worked and was an easy port, it would be wasteful management to devote funds, resources, and man power that they didn't have to port it to steamworks when they already had GFWL as their netcode.
Also I wouldn't get too invested in this yet. So far the only solid announcement that's been made is that GFWL Marketplace is shutting down.
The statement that was let slip on the AoE site has been removed, and there have also been a few fake things thrown around regarding this statement.
While it's not impossible that GFWL might be closing up (it's not like MS ever really supported it, they were woefully understaffed and unmoderated) until I hear an official statement I'm going to take this as a miscommunication of the Marketplace announcement (AoE wouldn't be the first to misread that, half the bloody internet didn't bother to read past the word Live to the one right next to it - Marketplace).