I'd like to say that it would be neat if the Andalites had burst in guns a-blazin' to save the day. But, in my opinion, that would sort of be out of character for the Andalites. It is mentioned several times throughout the series, if I recall correctly, that the Andalites are best known for being arrogant. It doesn't seem like blazing to the rescue would really be their style.
Also, don't they have a semi-low opinion of humans anyway? They see us as primitive, and coming to Earth's rescue probably wasn't high on their priority list. And it does take a long time to travel out to this neck of the woods for them. Visser Three states in the first book that Elfangor is the last Andalite in the sector. (Not counting Ax, of course)
In the TV series at least, I don't recall about the books, Visser Three also makes the snide comment that the Andalite forces are spread much to thin to do anything about Earth. If this is true in the book as well, then it could be that the Andalites may, just may, have wanted to do something, but didn't have the troops available as they were probably defending other fronts. Such as their own home planet.
Another reason the Andalites don't come blazing in. That law about sharing technology. If they came down, then I'm pretty sure people would see them. They'd be exposing their culture and technology to a wide range of people. And that's sort of what they're trying to avoid, isn't it?
I also agree with the fact that this book series was trying to stay close to the real world. I felt it was easier to connect with some aspects of it. And ya gotta face it, people blazing into save the day right when you need it doesn't really happen in real life.