***** NOT A TEACHER *****
Hello, Summer:
Some books say that some speakers (and especially writers) like to reserve "as ... as" for positive statements and "so .. as" for negative statements.
1. I am as tall as George.
2. I am not, however, so tall as Tony.
3. You may stay with us as long as you wish.
4. Aunt Mona stayed with us for one month. But you did not stay with us so long as she did. Why?
*****
There is a theory. I do not know how accurate it is.
The idea is that native speakers are so accustomed to hearing "as ... as" for positive sentences that some speakers / writers thought that by using "so ...as," they would be emphasizing the negative to our listeners / readers.
Whenever I have time to think, I try to say sentence #2. (But in hurried conversation, I have no doubt that I would forget to use "so" -- because most people here in the United States use "as" for both positive and negative statements. And that is what I have heard all my life.)
James