With only one company using BD+ and not even on all titles i doubt BD+ was a big factor. If the other studios were interested in BD+ they would use it.
The following is ONLY speculation and my personal thoughts:
Knowing that your customers might be very angry at you for using
BD+ would you want to be the first studio to use it? Or second? Or third? What if it didn't work out so well and although they had tested it before releasing it for use it turned out that problems arose when it was actually released for public consumption. I don't know about you but I wouldn't have wanted to be the first studio to put my head on the chopping block and possibly end up losing it.
FOX stepped up to the challenge. Why? Maybe they were
"compensated" for being a guinea pig. I'm
not stating this is a fact. I'm merely throwing the idea out there. It's already known that both sides financially compensated studios for supporting one format or another. It's not a big stretch to then say maybe
FOX was compensated for using
BD+ to show that it worked.
Now also take into account the cost of
BD+. It was a publicly untested protection mechanism and it costs money to even license to begin with on top of the license fee for
AACS which is already on the disc. Again I must ask, would you want to be the first studio? Not I.
It's always possible studios might be sitting back and seeing what
Slysoft does with
BD+. If
Slysoft defeats it then why waste the money licensing it for your releases? That would be wasted money, IMO.
Lastly, regardless of the fact that
FOX is the only studio to use
BD+ it is an optional method of protecting a studio's content. The fact that it exists very well could have swayed studio support even if they haven't embraced it and haven't actually used it yet.
I believe
BD+ played a role in winning the format war but you can't discount the mandatory use of
AACS, as well. No one other than
Slysoft has managed to overcome
MKBv4 so it's not like
BD+ is really currently necessary. Burning a backup is financially unfeasible at the moment and the people I have talked to who actually do want to make backups are not your everyday normal person but are instead
quite knowledgeable.