[CydeWeys]

The 757 didn't sell nearly as many planes in total as the 737. There's entire airlines (e.g. Southwest) that only use 737s and have their entire business built around them. The 737 MAX was designed for this use case; a newer 757 might not have sold to these carriers at all.

Plus, the 757 has some features on it that make it more of a longer range, transcontinental jet than the 737. That makes it more expensive. If all you're using it for is regional air service (the existing mission of the 737), then it's too expensive for the job. The 737 is the Honda Civic of the skies; the 757 is more like a (insert more expensive car model here).