I think it's quite clearly the second part of the title. If it was just "How to Win Friends" it might be something more people don't dismiss just based on the title.

"... and Influence People" makes it sound like that's the purpose of befriending someone, i.e. getting them to do what you want, or to do something for you.

It seems to be a rather brilliant piece of marketing to put that phrase in the title. It raises curiosity in a way that a generic "make friends" does not. (The "win" is a subtle move also.)

Once the readers are drawn in, whether from base or nobler instincts, the book can try to influence its readers into being nice.

Only trouble is that it may push away those who are "already nice" enough to feel bad about manipulating people.