Alexis? Sanches is at Bayern, I think you're talking about Sánchez.
If it's Alexis then I've spoken about this many times before... He was surplus to requirements and wasn't providing anything extra to what Pedro was. Both players were battling it out for exactly the same position and both were being rotated by various coaches from Guardiola, Tata Martino to Luis Enrique. Alexis was the first victim and then came Pedro who went to Chelsea.
Plus Pedro being a La Masia player, he was more loved by the fans. Not that anything untoward ever occurred by the Barça fans toward Alexis... He was very liked in the dressing room and the fans liked him too. But the same as anywhere else, homegrown players have an extra connection with the fans.
Put it this way... He never displayed the form he has done with Arsenal. Now whether that's to do with the quality he faced in both leagues or whether it's to do with playing more at Arsenal, I don't know. Right now he's definitely a world class player and one that would fit into almost any team in the world.