Cause he's not as good as the hype made for him said he was, plus injuries.
Barca have had a bit of a lean spell with their La Masia talent in general being hyped and not delivering. Alena, Puig, Munir, Nico, Cuenca, Miranda, Perez, Ruiz, Denis Suarez, Samper were all variously highly rated there over the last 5-10 years and done nothing at Barca. Fati's probably had more of a chance than any of those and shown a bit more, but I can't imagine they'd loan out a Gavi for instance if they rated him that highly, this just sort of seems more like "you're not progressing as we'd hoped, here's a loan".
Plus they have that other kid now, Lamine Yamal, who only turned 16 a month ago and is already starting La Liga games for them.