Messi lacking confidence in Barca's board and management = he's going to look for something he has confidence in.
The only other person he's worked with before, in world football, in a club that is doing well and has a shot at global honors, with whom he's had plenty of success before.. is Pep @ City.
If we're talking about other options with strong existing teams that have potential global success without a working history with Messi that'd be Zidane @ Real (impossible), Klopp @ Liverpool (if they ever wanted to pay that much), Bayern, PSG and Juventus (unlikely given $$ with Ronaldo on payroll).
We belong to the next tier down -- we can afford him, but our prospects aren't as immediately bright as the teams above. I'd be overjoyed if Messi joined us, as that'd likely mean winning the PL next season. But really, why us instead of Pep/City? No brainer.
(Lastly, I don't believe in the "history" argument.. if we all believed that, there's no room for football to grow.. it'd be forever trapped in haves and have-nots. There have been plenty of clubs with history that have fallen by the wayside e.g. Nottingham Forest. Whether we like it or not, and whether the source reason is just or not given the dark oil money and flagrant rule breaking.. City is history in the making. Not huge history, but relative to where they've started from.. yeah, it is)