Because you use it that often. Or at least, I do. Maybe you're a much better typer than I am, and for that matter, much better at correctly typing what I want in the first place, but I consider it an important key.
One issue with using statistics to layout your keyboard is that you if you analyze your layout via text corpus, you'll come up with a usage of 0 on the backspace key. This is inaccurate. You really ought to do an analysis based on actual key usage, but that data is much harder to come by. (You'll probably just have to take your lumps on the possibility that changing the layout would itself change key usage.)
Oh, and per other comments on the difficulty of trying out new keyboard layouts... rewriting how you do all the letters is very challenging. But if you are interested merely in trading Caps Lock for backspace, you can literally try this at any time; adjustment will take on the order of 5 minutes, tops, as long as you unmap the original backspace key. And it's about a minute to switch back if you don't like it. Professional keyboard users really ought to be people who have tweaked at least a key or two, if not tried entire layout switches. It's so fast and easy to run the experiments.
Because you use it that often. Or at least, I do. Maybe you're a much better typer than I am, and for that matter, much better at correctly typing what I want in the first place, but I consider it an important key.
One issue with using statistics to layout your keyboard is that you if you analyze your layout via text corpus, you'll come up with a usage of 0 on the backspace key. This is inaccurate. You really ought to do an analysis based on actual key usage, but that data is much harder to come by. (You'll probably just have to take your lumps on the possibility that changing the layout would itself change key usage.)
Oh, and per other comments on the difficulty of trying out new keyboard layouts... rewriting how you do all the letters is very challenging. But if you are interested merely in trading Caps Lock for backspace, you can literally try this at any time; adjustment will take on the order of 5 minutes, tops, as long as you unmap the original backspace key. And it's about a minute to switch back if you don't like it. Professional keyboard users really ought to be people who have tweaked at least a key or two, if not tried entire layout switches. It's so fast and easy to run the experiments.