It's not about the shape of the bars, it's the weight, stiffness, and portability of the frame, the size of the wheels, the weight of the tires, and the handling characteristics.

Here's a thought experiment: Build the ideal bike for racing CX based on an MTB. It'd be a 29er hardtail, so the tires would roll faster. You don't need any suspension, which just robs your energy on climbing and accelerations anyway, and weighs down the bike. So, rigid fork. For the frame, you put enough room under the main triangle so you can shoulder it when necessary; this is important for courses with unrideable sand or mud. You trim weight on the tires to give you an optimal trade-off of weight to traction. Based on these smaller tires, you can alter the bike's geometry so it handles more nimbly.

Oh look, a cross bike.

Sure, there might be sections of a course where you'd prefer an mtb. There are minor tweaks outside of UCI rules, such as 35-38mm tires, that might make sense on some days or some courses. You personally might feel more comfortable with flat bars. But on balance, and by far, a cross bike is what you want for going fast.