Because squat is generally considering the best single exercise you can do. The barbell squat hits basically every major muscle group apart from chest. Your neck, entire back, shoulders, stomach, hips, quads, hamstrings, glutes and calves are used in the squat. It's the best way to load your entire body with heavy weight and elicit muscle growth and strength gains for the entire body.
Deadlift is a close second, but squat is just a little bit better for your entire body and the movement is harder to learn. The best way to get better at a lift is to do it often so you squat several times a week.
There's a lot of overlap in the exercises too. They largely target the same muscle groups, and you need recovery time. The program simply works so stick with it!