Because Jones is younger, taller, stronger, faster, more experienced, and better at everything except perhaps submissions.
And he's a LHW! Why is it so hard to understand that weight-classes exist for a reason?
That still does'nt mean Weidman wouldn't have a real chance at beating Jones. I would pick Jones to win as well but let's not forget Belfort moved up to LHW and almost submitted Jones. Weidman just made quick work of Anderson Silva which is certainly nothing to scoff at. I'd most certainly say Weidman would have a real chance at beating Jones.