The characters are selected because they possess specific characteristics that make them ideal to handle various cases.

p possesses maximum "descent" for a character (the bit below the baseline on a font).

É not only meets the traditional "ascent" but exceeds it since diacritics normally extend past the traditional ascent on a capitalized letter.

These aren't the only characters that possess these values (although, I assume some selected code points are unique), but just ones that do.

Presumably because browser were historically much better at handling and rendering English and Western European characters (and input) than they were at other scripts?