I think it was a combination of factors. Part of it was the fact that, yeah, Tobias didn't really quite completely want to be human again. There was a part of him that did, and a part of him that didn't, and I guess the Ellimist decided to listen to the part that didn't. P.S. I never really believed the whole "oh, a tiger/horse/gorilla/elephant were able to get out of the Yeerk pool but the hawk apparently wasn't stealthy enough to give the guards the slip" story. I definitely think there was something subconscious going on there.
But, also, the Ellimist knew that Tobias needed to remain a hawk. Remember #33? The Anti-Morphing Ray, and how they got rid of it? The Animorphs absolutely needed Tobias-as-a-hawk for that mission. Any other Animorph would have been revealed by the Ray and infested. And they would not have had the prior warning that they were lucky enough to get in #49. No, this would have been a swift and sure annihilation of their resistance, quickly followed by the downfall of Earth.
They couldn't have just destroyed the Ray, either. A weapon that powerful, and that critical to the Yeerks, they would have rebuilt it. No, the Animorphs had to prove to the Yeerks that the AMR didn't work at all, that it was just another failed experiment, useless to try again. Without Tobias, the Animorphs would have been forced into hiding, unable to attack the Yeerks, for fear of being exposed to the Ray.
It's been said that the Ellimist doesn't see THE future, only possible futures. But he may have known, even then, that the Yeerks were intent upon finding some kind of forced-demorphing technology. And he may have known that the Animorphs would need a way around it. Who better than the nothlit for that role?
Actually, I even tend to think the Ellimist may have been thinking even farther ahead. The events of #33 may have been why the Ellimist was apparently so keen on Tobias being born in the first place. Nothlitism runs in the family, heh.
