My guess wages and marketing power. Though I can't figure out the latter, since there is a direct conflict with both PSG/Nike and Neymar/Nike. Pogba switching gear sponsor seems out of the question either. Maybe squad player for national quotas...
He isn't going to be a galactico, in fact Spain as a whole is a dab-immune zone. Coming here means lower wages and more importantly a confrontation with delusions of the past and disappointments of the present. PSG is the only easy escape strategy and a ride on the luxurious train towards
CL glory. Either way the Jose/Pogba, Manchester/Pogba 2 fallouts would be EPIC!!! Another vacation? Visiting mum? ...