He doesn't have the consistency required to play in midfield. Because he doesn't make the consistent very good pass, he either plays it safe, or tries something fancy and loses the ball. Xavi was not that erratic for example. He almost always played the safest very good pass when in CM position. It was Messi and even Iniesta who did more of those tricky high-risk passes, but then, they played higher.
You could even spot Xavi during a game, thinking about passing somewhere, but just not doing it at the last second because the passing lane was too tight and the outcome of the pass would've been too uncontrolled. This is everything about being a great playmaker at Barca in a deeper position. Finding a balance to provide very good passes and doing that with 88-94% success rate.
For Xavi, playing the safest very good pass most of the time in his career is one of the reasons why he is, undeservedly, rated lower than a player like Zidane for example by most football fans.