It's because he didn't have the team spirit, which led to a lot of conflicts between him and the team, and then he chose to leave Barcelona to join Manchester City, but during his two years at Manchester City he wasn't recognized by the team manager, which led to two seasons in which he didn't get any chance to play. So his talent was slowly running out and he wasn't wanted by any team at this point in time.
Guy Assouline is a young player from Israel who showed great soccer talent at a very young age. And the youngster has been on Barcelona's radar since the very beginning. His ability to play in Barcelona's youth system has led to many comparisons with Lionel Messi. But this has also seen him endure some very big names at a very young age. But that fame didn't make him any better.
Later on he didn't put his whole heart into his training, which made his ability later on worse as well. What's more, Barcelona is very much a team player. But he likes to fight alone, which also seriously undermined the tactical system of Barcelona's playing style, which also let him gradually be marginalized by the Barcelona. Later, he chose to leave Barcelona in order to find his former status, but later he did not find a suitable team for his development.
Messi's talent is surprising, but for a young player, only hard training can bring out his strengths, and the young player did not go through the rigorous training, which led to the level of their own and their own show of talent is far from the difference.