United's dream is to keep Van Gaal until Giggs is ready to manage them, then for Giggs to be the next long term manager, playing football 'the United way'. In reality I think they'll cave and hire Mourinho at the end of this season.
This is the one and only reason. No chance Mourinho will manage Manu.
That's only us available and he knows it full well. The only stumbling block is his wages and transfer fund to convince him imo.