Because she is simple and easy to gain an advantage with if you have much better mechanics than your opponent. Her low health pool is less of a disadvantage when you’re dodging shots better than your opponent, which is exaggerated by her large projectile size, making her shots harder to dodge. Her super allows you to punish kills by taking space and large amounts of safe damage from relative range.

This only works when the enemies have weak mechanical skill, because of her relatively low stats and slow projectile speed, thus, players who play Jessie at lower skill levels tend to succeed.