It's not a matter of being commercial or being heavy with marketing, it's for practical reasons. Two new kits are coming out every season as it is. The other part is true, though. I think it was probably a case of Rosell running his mouth and promising something that was out of his power. Nike imposes new home and away kits every season, and if the previous season's away kit turns out to be too similar to either one, common sense (or should I say, the FA) dictates you're not going to use it.
The senyera away kit has been a historical commercial success for the club and Rosell said it will be appearing more and more in Barça’s future. “It’s very beautiful. The commercial sales have been impressive and it will continue as the third uniform of the team next year and also for the time we stand as the club board“. The uniform will also be used in “special” matches.
Then again, you could argue that "the time they stand as the club board" has passed.
IMHO, what they should have done, if it was possible, was to push for an away kit that was not similar in color to the senyera. Maybe the lime green one. Then they could have kept the senyera. I doubt Nike has anything in principle against keeping the senyera around, especially if it's selling so good.