As you have said it yourself, this is a matter of usage. You don't argue with usage. Faced with usage, ours not to reason why...
As an example, you commonly say "make a mistake", as in:
Don't worry, we all make mistakes. (Oxford Dictionary)
Now imagine you want to correct something people have said, and even though you are quite confident about it, you are trying to be polite. Will you say:
If I haven't made a mistake, the emperor has no clothes.
Well you could. In the same way that you could say "how expected", but that's now how some English guy decided to put it hundreds of years ago. He/She went for this:
If I'm not mistaken, the emperor has no clothes.
And that's the usage now. It's the "more correct" form. It sounds and feels better.
Having said that, there's usually a loose logic running through the fabric of the language that we may sometimes manage to catch hold of.
In our specific case, I think the key lies in the "tone":
"How unexpected" is a simple expression of surprise.
"How predictable" is sarcastic.
That's why "predictable" fits in much better:
expected: that you think will happen (Oxford Dictionary)
Double the expected number of people came to the meeting.
predictable: (often disapproving) behaving or happening in a way that you would expect and therefore boring (Oxford Dictionary)
The ending of the book was entirely predictable.