Well, there are actually a lot of diatonic accordions doing just fine, in other cultures. France still has plenty of diatonic accordions, ( for bal folk) , as well as Quebec, Slovenia, Austria , Belgium, Netherlands, Italy, Germany.... Diatonic accordion can put out a rhythmic drive that can be hard to duplicate on a piano accordion, and that makes it still thrive for dance music. Listen to some Tex-Mex two-steps by Flaco Jimenez, or Cajun music by Iry Lejeune , and you might see the point.

Bi-sonoral accordion systems like the diatonic accordion came first. Mono-sonoral systems like piano accordion or chromatic button accordion ( like the French accordion ) came to be popular in the 1920's, because musicians wanted to do more complex melodies with their right hands and more complex harmonies for them with their left hands. It's challenging to play a lot of the French muzette repertoire on diatonic accordion, now (the music you often hear in the background when someone is trying to set a film in Paris) even though the earlier tunes, by players like Emile Vacher, still work, because they were written on diatonic accordion.

Though diatonic accordion tended to have fairly few choices of left-hand harmonies- a piano accordion has a full Stradella bass, rows and rows of buttons- Irish "box" players are fine with having few because they often don't worry about playing the bass or harmony...the older tradition in Irish music is for everyone to be just playing the melody, anyway.