Thinking about how one of the first things we learn about Cassio is that he's from Florence.
Obviously, Othello, at its core, is a play about racism. Othello sticks out because he is a Moor and one in a high position in society at that. Cassio would definitely be given more leniency because of the racism. This is just an add-on to that and the theme of "otherness" in the play. (Also, Iago is already racist and sexist. Xenophobia would not be out of the question. There is no "too low" for this man)
I'm just saying you have this other "outsider" in the sense of Italy not being a unified country and just being composed of city-states/republics/kingdoms/etc. Everyone spoke their respective dialects, and it would take a long time for them to become one nation. The idea of one of the layers of Iago's hatred being that these "others" moved up in society before him AND on top of that he has to take orders from them is very on brand for him. Cassio didn't have to be written as a Florentine, but the fact that he is just adds to Iago's hatred and jealousy because he has to obey people that he sees as "lesser than."
Also, Cassio's mannerisms and courtesy get twisted by Iago into acts of flirtation. Iago repeatedly uses stereotypes to convince people what he wants them to believe. He takes advantage of courtesy and kindness and vilifies people in order to get his way. A big part of why Othello falls victim to the manipulation is that Iago utilizes the insecurity of being a black man in Venice when convincing him Desdemona is unfaithful. Cassio, being a man of high manners and courtly to women, makes that worse because Iago uses this to "prove his point" in a sense, playing off these biases to get what he wants.
Footnote: Florence was also an intellectual capital at the time, which ties into Cassio being an arithmetician (and the other reason Iago hates him so much)
