Why was peeta hijacked?

Peeta was hijacked because President Snow wanted to turn him into a weapon to use against Katniss Everdeen. He knew that Katniss was Katniss's weakness and that if Peeta could gain Katniss's trust and then betray her, it would devastate her. Snow also wanted to use Peeta to manipulate the public opinion against Katniss and the rebellion, and to show the world that even the strongest and most beloved rebels could be turned against their own people.