In short
At the shore of the Sea of Galilee he could speak to more people than in the house (Matthew 13:2). At first Jesus taught in the synagogues, but later mostly taught outdoors. While the synagogues were not yet closed to him (see Matthew 13:54), they soon would be.
In Matthew 13:1 we read that Jesus went to the shore of the Sea of Galilee because there he could speak to far more people than in the house (Matthew 13:2). At first, Jesus taught in the Jewish synagogues, but later he mostly taught outdoors. Even though the synagogues were not yet closed to him (see Matthew 13:54), it would not be long until this would indeed become the case.1
1 That same day Jesus went out of the house and sat beside the sea.