User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-4902593-20121229113429/@comment-3146930-20121230120651

A lot of the details of the first Gigantes War aren't really explained. There could have been other heroes that aren't mentioned or something. You also have to consider that the gods are weaker now, meaning they are the ones that need more help this time around. They are also still recovering from their war with the Titans, as Hephaestus said.

Lastly, after being defeated the first time, Gaea most likely has been thinking of new ways to defeat the gods. She presumably has learned from her mistakes and is deciding to correct them. That could mean that more demigods are required because the enemy is more dangerous.