I think the main reason was the bad relationship between Laporta and Xavi otherwise he could stay for the next season.
Even Flick was not successful recently and they could have much better coaches instead of him
Xavi was expected to be kicked from Barcelona at any moment especially that few weeks ago he was declared out then resumed... Their might be some misunderstanding and tensions between him and the staff but that was mainly due to the poor results they had and for not winning anything this season.
That's freaking bullocks and an extreme shit for Barcelona board, It's really an awful way to treat a club legend, frankly!
Xavi had initially voluntarily decided to step down at the of the season, and he was implored by the board to stay, which he later agreed due to his love for the club, both as player and now as a coach, if anybody had been able to repair the relationship between the club and its fans, it definitely was Xavi, and he sure was the fan favorite. The board has successful setback the club backwards by a couple of years with this move, except they expect Flick to come and perform a miracle on limited resources and budget.
Who freaking pleads with a coach to stay on, only to have it thrown back at his face a few months later, BS!