Because Barcelona are still able to keep the gap between them quite balanced. I don't think they would maintain a very bad form in the league while their main focus is on there already. After a break of a few years in the league Barcelona finally have a big chance to become the champions. They wouldn't gave it away easily. Their simple mistakes are happening less often than Real Madrid during the season.
If they would want to do it is not at this time that they are already close to Success.
The major factor that can make Barcelona to lose the title race to Real Madrid is injury.
But looking at the current squad of Barcelona, you will understand that almost all the players are available.
This will rather make it difficult for Real Madrid to penetrate.
Real Madrid should simply taking the Champions League and forget about the La Liga which Xavi has worked for