Because they've won no major European trophy.
If you haven't won the Champions League, or the European Cup before it was commercialized, you're a non club.
Even the Europa League is a trophy for losers. It's basically a trophy that says 'Look at me, Look at me, we're a bunch of losers who couldn't win the Champions League so we won this trophy to pat ourselves on our backs and feel better about ourselves'. It's hilarious.
There's only a handful of club trophies worth winning:
Every other trophy is a joke, including the League Cup in England.