Influence
The traditional Sabbat view is "Don't play with your food." Talking to lesser creatures makes you look weak in the eyes of the Sabbat, and you're likely to be less trusted, and a little too Camarilla-like. The Sabbat aren't very pro-weak, untrustworthy Camarilla-like people, and openly garnering and using influence is going to get you in trouble.
That's the traditional view, the official standpoint. However there is a growing feeling in modern times that "We're fighting a war, use every weapon there is.* Just don't get too involved and don't get caught."
Also, if you're not on path of humanity, interacting with humans is very difficult. To get on a path, you need to completely surrender your humanity - forget what it is to be human, and find another way to anchor yourself against the beast. You become utterly alien to any mortals, and it's pretty obvious. You now longer do the little things, the non-verbal communication. Your sense of personal space, posture, mannerisms are all lost to you. You can't spend blood to activate organs.
In short, unless you've been specifically taught how to relate to humans (acting skill, etc.), you will find in exceedingly difficult to get people to interact with you. Furthermore, you're weird and will tend to make a lasting impression on someone. In short, if you're not on path of humanity, you will have a really hard time getting influence, a harder time keeping hold of it, if it gets contested, and you won't be able to use it as efficiently as someone with Humanity.
*Except Daemons. The Sabbat hates daemons and all things infernal.