Shift+PageUp, Shift+PageDown have historically been the shortcuts for scrolling, hence application don't expect these keypresses to get delivered.
Shift+Up, Shift+Down, however, generate escape sequences that are useful for applications (e.g. select text in editor), it would be bad if these key combos were handled by the terminal itself.
My 2 cents:
Shift+PageUp, Shift+PageDown have historically been the shortcuts for scrolling, hence application don't expect these keypresses to get delivered.
Shift+Up, Shift+Down, however, generate escape sequences that are useful for applications (e.g. select text in editor), it would be bad if these key combos were handled by the terminal itself.