What makes egoshiping so appealing is because it's crack. Fan fiction writers and artists like to experiment and be more creative, for example use a pairing that viewers, wouldn't see happen in the actual show.
Misty is also a very popular character in the pokemon fandom. The same goes with Gary although he has not been in as many episodes as Misty. Many like to pair their favorite female character with thier favorite male character. So that is one way people may like egoshipping.
Ash/Misty (pokeshipping) is a very popular shipping on pokemon. Gary is Ash's main rival, so this is where the pokeegoshipping love triangle is kinda formed.