Why do you one trick a hero?

Hey, curious about why you one trick a hero? Some reasons I can imagine is a disability (srs) ... IE i think I've seen a thread where someone said Moira is all they can play because of a disability.... but what are some other reasons?

Another i can think of is character attachment... I know kids used to LOVE playing genji for some reason, and will just sit on that character for the whole game despite the enemies playing a full on lazer comp .... In open queue, there are 2 names I see often on playstation in higher elos who always (and only) play pharah... So if they are on the enemy team, i already know who to pick to make them rage quit....

Whats your reasons for one-tricking a hero?