Why is it unnatural to say "He is as clever as rude"?

(1) He is more clever than rude.

(2) ??He is as clever as rude.

I've learned (1) is natural and that (2) isn't.

(1) means "He is clever rather than rude".

Then, why can't (2) mean (2^b)?

(2^b) He is clever as he is rude.

In other words, why is (2) unnatural when (2^b) is natural?

EDIT
The question used to ask why (2) is ungrammatical, but people here seem to agree that (2) is grammatical but just might be unnatural in Present-day English.

Questions

1. I'd like to know why (2) is unnatural, not whether it's unnatural.
2. Also, how natural (1^b) is and why?

(1^b) He is more clever than he is rude.

Addendum

The Cambridge Grammar (p 1121) says this is grammatical:

(3) Ed is more old than middle-aged.

and that this is not (p 1122):

(3^b) *Ed is more old than he is middle-aged.

So, the Cambridge Grammar specifically says that the insertion of he is in (3) is what makes it ungrammatical.

BTW, the book calls this construction metalinguistic comparison.

“He is as clever as rude”?

It's not actually clear to me that all English speakers would agree that “He is as clever as rude” sounds unnatural. In fact, the following parallel example is treated as grammatical in Longmans' School Grammar, by David Salmon (1890):

The teacher is as clever as kind. The teacher is as clever as [he is] kind.

(p. 162, accessed through Google Books)

On the other hand, "The teacher is as clever as kind" and "He is as clever as rude" do sound a bit weird to me (and Mari-Lou A left a comment indicating that this construction sounds bad to her as well). Also, I did find a WordReference forum thread where more speakers mention that omitting "he is" in sentences like this sounds bad to them: She is as beautiful as smart.

To sum up, it seems the acceptability of this construction might vary between dialects or time periods: if so, any explanation for why it is "unnatural" would have to at least allow for that variation (if not account for it).

A Feb 13, 2018 post by entangledbank in the WordReference thread suggests that it is generally not possible to omit a subject, or a subject and verb, in comparative constructions like this. This would suggest that “He is more clever than rude” is not actually an elliptical form of "“He is more clever than he is rude”; this is consistent with my intuition about that sentence, but maybe not with other speakers' (see discussion below).

“He is more clever than (he is) rude”

I'm not sure about the grammar of (1); to me, "He is more clever than rude" sounds better than "He is more clever than he is rude," but Jason Bassford has left a comment indicating that he has the opposite judgement.

My guess would be that it feels unnatural because people expect a comparison with another subject:

He is as clever as Tom.

So when we hear 'as clever as rude', it sounds confusing and potentially ambiguous, so we repeat the subject for emphasis (as noted in your question)

He is as clever as he is rude.