Because Pogba absolutely doesn't want to miss UCL games? Like any other normal player? It would be unlikely for even a hardcore cule to do that for Barcelona, and you probably know that Pogba is almost a mercenary. He only cares about the money, RM or Barca, he doesn't care.
Really ??
Do you really think he'd worry about playing in a few CL group matches if it meant he could join and start playing for us in January .... rather than risking the whole deal for 12 months - a serious injury could scupper his move to us next Summer .