It's a fanbase and brand name issue. It takes 10-15 years of continued success and a smart, aggressive business strategy internationally. Clubs such as United, Liverpool, FC Bayern, Barcelona, Real Madrid, Ajax, Milan AC and Internazionale all have much wider global name recognition and support than the modern upcoming clubs like Chelsea. They finished 6th in the league this year despite winning the Champions League.
I think they will get there eventually, but these things take time. If they're still in and around the mix at the top domestically (which they havent been this season) for the next decade or so then we'll start to see them accepted as one of those european superpowers like the clubs mentioned above.