Let's be real, here.
You can't make your players do anything.
"You can lead a horse," and all that.
You may need to come to terms that they simply won't do it.
Once you have done that, I'd suggest starting by talking with them about why they won't do it. I mean, if they didn't grab up the extra Karma they must have a fairly decent reason for not doing it. Either that, or you didn't offer enough.

I suspect you will find your players just aren't into that.
All by itself, that is not a bad thing.
The real question is: "Is your players characters lacking a background causing a problem at the table?"
If not, practice your "zen duck."
"Bead up. Roll off. Bead up. Roll off."
And just go on with your game.
If it is causing a problem, that is something else entirely.
Anyway, assuming it isn't causing a problem, with that, make sure you take a little extra time to prop up your other players backgrounds. More RP time with contacts and such. Maybe their contacts come through a little bit more often. Show off what a solid background can do.
And if your players
still don't want to make a background, let it go. Different goats for different folks.