Because at some point Marco pointed out "We give in now ,they own us."
The Animorphs, humans, won the war, and in the process made a number of discovers that the Andalite military would have keep secret. And during those neogiations, the Animorphs proved you can't just tell the humans to keep these secrets and expect it to happen. Earth may not be complete equals, but they were going to allies to the Andalite people, not the military alone, and the soldiers are going to have to slow down their dealings less the people and their allies turn on them.
The other is the promise that it's much more possible to take the HB planet than Earth. The Yeerks invested too much into Earth, and the Andalites finally, seemed, to get their sh&t together and stop fighting on all fronts. They lost HB world in the 60's and probably never tried to take it again, but now that they have the Yeerks on the rope it's reasonable to reinvest there. The council probably isn't there directly, and forces not equipped to handle a full Andalite onslaught, and, hopefully, someone is insisting that the Yeerks there get the same offer as the Yeerks on Earth, the nothlit route.
It's political suicide to do it knowing it would be disapproved of, suspected, found out, and logisitically there are alternative now that they didn't have/see before