Comment by AngeloR

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.