The second reason Midoriya calls Bakugo Kacchan is due to respect. Even though he was constantly abused and beaten by Bakugo, he admired Bakugo secretly.
He loved his tenacity, his quirk, the way he could handle it and everything about it. If there was no All Might Midoriya would’ve probably admired him.
The third and final argument is probably the closest answer to reality. It’s Midoriya’s way of coping with abuse from Bakugo. People in abusive relationships keep on telling themselves, no matter what things get better with time.
The same can said of Midoriya, he doesn’t want to let go of the happy picture of Kacchan created in his head and hence he subconsciously addresses Bakugo as Kacchan.
What’s your theory regarding this? Let me know via the comments section, I’d be more than happy to hear them from you guys 🙂