I think partly because it's a national forest, so it's protected ground. also it's not just a patch of woods near cassie's house. It's a big place. After many encounters where not only were they not only unable to kill any andalite, but the andalites actually found a way to significantly hurt them (finding and destroying the kandrona generator), they decide they need to start being proactive in finding this group. I'm not really surprised it took nine books for it to come to a head.
Also they most likely started this operation earlier, and the Anis just found out about it then. It does take time to set up a lumber camp, after all.
