Waiting this long to give the backstory to how he got OP may have been a "marketing" decision on the mangaka's point.
If you front load the exposition in a new series readers might lose interest before getting to the exciting bits and lead to an early ax. In that case, start off with the "hook" to build engagement and once you have a more reliable readership you can slow down and fill in the narrative gap.
I think I'm pulling that from reading Bakuman a long time ago so I could just be going senile. 🤷♂️