'To' vs 'in order to' in negative clauses

Solution 1:

It's true that so as not to and in order not to are much preferred among the majority of English speakers. I couldn't (without some deliberation) go so far as to say that not to is ungrammatical, but it sounds wrong to me, i.e. not fine.

As to why it's not consistent from a positive context to a negative: that's a good question, and even a broad site like Wikipedia doesn't address it:

Full [Infinitive]

It can be used like an adjective or adverb, expressing purpose or intent. So, "The letter says I'm to wait outside", or "He is the man to talk to", or "[In order] to meditate, one must free one's mind."

I googled infinitive purpose negative and came up with pages like this and this, which say essentially that we use so as not to or in order not to, rather than just not to, without explaining why: