No one knows for sure, but one thing that seems pretty clear is that he has an aversion to media scrutiny, and would likely prefer a "low key" home city.
It's been speculated and rumored that he flat out would not play in NYC, and I have to imagine that's a big part of why. Chicago isn't NYC, but it's not LA or Toronto, either. Additionally, it's been said that Shohei is a creature of habit, and probably was never really looking to make any radical changes from the life he's led stateside since he first arrived.
And then there's the weather, but who knows hoe much he cares about that.
It's all speculative, but it seems a given pretty much all the major players here were prepared to give him $500-600M, and at that point, it really comes down to the other stuff, and I've felt the whole time like Jed & Co. were sailing into the wind here even if they were willing to pay.
EDIT: All that said, I am still clinging to hope here, because I really feel like the pursuit has been sincere. I'll let it go when it is reported he's signed somewhere else.