Well, I assume a 5'8" 3B isn't going to get to as many line drives (whether jumping or diving). My guess is Harrison, especially as he's aged, just doesn't grade out well enough as a 3B, only 37 starts in the last two seasons (all this past season). He'd be great to just have on the roster without trading Franco but if he's disgruntled in Pittsburgh, coming to the Phillies, even if they are on the rise, w/o no guarantee of starting, probably doesn't appeal to him either.
But remember - Machado wants to play shortstop. I seriously can't imagine a scenario where he's not a shortstop next year, if teams are bending over backwards to woo him and there are multiple teams offering the same $, that will close the deal.