I know you asked this a very long time ago, but I though it was interesting. I've confirmed with coworkers; one is an English teacher and the other is a linguist. The answer is that there technically isn't anything wrong with the second example. But if course you're probably asking why we just "know" it sounds weird, and therefore use the first example without thinking. The answer that they agreed on is that "earlier" tends to come with a phrase attached, e.g. "earlier than 7 a.m." and such particles tend to be found at the end of a sentence. Of course in this case it's not. But generally, "are you available earlier?" would be implicitly referring to a time already stated. "I can meet at 7 a.m." "Are you not available earlier [than 7 a.m.]?" It's this "expectation" of the particle that makes the sentence sound strange if it's anywhere but the end. But you're not breaking a grammatical rule by saying it either way.