Any difference between "as to" and "about"?

Question

Are these two expressions interchangeable? Or is there an important difference? example sentence:

Everyone was curious as to why Mark was leaving.

Everyone was curious about why Mark was leaving.

Answer 1

There are very, very few words or phrases in English that are entirely interchangeable – each word and phrase has its own nuance.

When speaking any language, people include intonation and gestures to add meaning.

Thus, as to and about are not fully interchangeable in all cases: As to is a free modifier that modifies the main clause as a whole, about is a bound modifier that modifies the last substantive.

1. A: I have explained the manufacturing process – as to the storage, I will leave that (i.e. the explanation of the storage) to Mr Smith. (correct)

2. A: I have explained the manufacturing process – *about the storage, I will leave that (i.e. the storage) to Mr Smith. (wrong)

You will note the referent of “that” in 2 is storage

As to X = as far as the subject of X is concerned; to the extent that the subject of X is concerned

About X = on the subject of X

Answer 2

Yes they are interchangeable and there aren't big differences between the two.
However, I prefer using about, because for me is more informal, but you could have used both.

Here you have a deeper explanation of the little differences that there are between those two:

differences between "as to" and "about"