It has less syllables so it’s easier to call out and it makes more sense that he’d be known by a nickname. Plus it sounds cooler. They don’t even need to change his official name of Cole Cassidy, they could just change the name he’s known as in-game to Deadeye. Then they could change the name of his ult to something else. Besides, didn’t they say he’s getting a rework in Overwatch 2? If so they should certainly reevaluate his ultimate since it is hard to get value out of and often feels like it is wasted. His ultimate could be improved upon to allow for more skill expression, at the same time remain an iconic and familiar part of his character, and remain a high-risk high-reward ultimate that emulates a standoff in a cowboy movie. Although sadly its probably too late to rename him again because they’d have to change it again in the next Overwatch art book and the voice lines and everything else.
Honestly the length of the name isn’t a huge issue. Much like people used to just call him Cree I am sure it will be just Cass before long. Still very odd any time some one calls out Cassidy or I see it in POTG etc. Call me crazy but just seems odd to change a characters name 5 years later.
Then again I was against the Sym and Torb reworks for simular reasons. They basically changed their entire character, especially Sym. Then for Lore reasons Torb went from loving and babying his turret into something that’s super disposable.
The MCree name change just felt off… and it delayed game updates further to a game that hasn’t had an update in 2 going on 3 years now.
McCole
McCassidy
They could have saved the “McC”
I still wanted Dave 
I literally don’t give a damn what stupid woke name they give Mccree, i’ll still refer to the character as mccree.
They gave the dude a girls name. lmfao
10+ woke points!
Okay. Good for you.
They didn’t, no.
I also fail to see the issue with this.
yawn
We still talking about Jesse McCree’s new name?
snore
McNuggets for sure 
I think its a reference to Butch Cassidy the cowboy
Nobody would call him deadeye or else it’d be insanely confusing when he’s using his Ult.
Nobody calls him Cole because it’s too easily confused with Moira’s Ult, Coalescence.
Cass is… the best we got besides Cree.
How do you not know Butch Cassidy and the Sundance Kid? It’s not only a guy’s name, it’s a famous cowboy movie, and it’s his last name.