Great Question.

I've always had to answer this to 'non believers'.

Because each member is outstanding on their own instrument.
Because of their ability to produce something special live.
Because of the chemistry between Jon and Richie; songwriting.
Because of the chemistry between Jon and Richie; onstage.
Because of the harmonies they produce, they are outstanding.
Because of Jon's ability to make that person in the 89th row think he's talking to them.
Because of Richie's and Dave's vocal range.
Because of Dave's ability to sing those parts of the harmonies no one else would even dream of (live).
Because of their stage presence.
Because I never get bored of their songs.