Because the pro used to be the largest iPad with a pencil. Now you can get the air instead and "pen and paper" certainly does not need "pro" performance.
As a physicist, the main reason why I chose the Pro version is the refresh rate. It doesn’t distract me when I’m using the pencil.
Also the keyboard when I need to quickly ssh into something and I didn’t bring my laptop. (Not sure if there are non-pro iPads that support them now)