First of all, many cellphone users don't change launchers.

The ones that do understand that launcher and lockscreen are different.

Facebook Home replaces both, and remove some major aspects of the Android UI that most launcher/lockscreen replacements DON'T change: things like getting rid of the notifications bar, changing the lock screen slider, changing the way that you get to apps

It's a very different imagination of how to use your phone. I'm not at all surprised that many people don't understand it and/or don't like it.