Why is it なさそう and not なそう

Question

"It seems there is none" is なさそう, which escapes the usual rule for 形容詞　(イ-adjectives), which says "drop the い and add そう".

Is there a historical explanation for this exception? And does it have anything to do with なさいません?

Answer 1

Your "usual rule" is incomplete. It should be:

1. drop -i
2. if resulting is a single mora in length, add -sa
3. add -sou.

Hence, nai:

1. na
2. na + sa
3. na + sa + sou --> nasasou.

atui:

1. atu
2. (not applicable)
3. atu + sou --> atusou.

Answer 2

Well, in first place there are possible confusions depending you are refering to ない as a [助動詞]{じょどうし} or 無い as a [形容詞]{けいようし}.

In the case of "It seems there is none", なさそう will be 無い adjective　[連用形]{れんようけい} + そうだ [助動詞]{じょどうし}。

It seems that when そうだ was first introduced at Muromachi period, なそう was used but as な is only one syllable, さ was added through time.

It has nothing to do with なさいません which is the negative form of [為]{な}さる verb.

For information, in the case of ない as a [助動詞]{じょどうし}, なそうだ is used:

彼はお酒を飲みなそうだ。

But recently, さ is sometimes added:

彼は来な(さ)そうだ。