It's quite simple. Messi went there because PSG was the only top team that wanted him and they are the team that can seriously compete for CL. Messi and PSG couldn't care less for League 1. It's all about wining one more CL. Do you think Ronaldo cares about another EPL title? It's all about wining another CL.
he went there because of multiple factors:
- psg showed intent and moved in quickly
- team is full of south americans or spanish speaking personnel(spanish is on par with french in that dressing room)
- has friends there - neymar and di maria
- they offered good money
- paris is more close to his heart in terms of lifestyle and closer to his home in barcelona
- psg lets him go play for argentina whenever he wants
- city were left surprised as they got grealish and probably opted to go ahead with plans of adding kane or another pure CF