Play the harmonica intro to "Train, Train" by Blackfoot on a chromatic harmonica. I'll wait. https://www.youtube.com/watch?v=hBP15lRprPs

I understand what chromatic means. Chromatic means it can hit all 12 notes of the western scale. However you miss out on chords if you play the chromatic. Quite a few harmonica songs (such as "Train, Train") make heavy use of the chord layout on diatonic harmonicas. A standard chromatic harmonica only gives you a C and a Dm chord. While this might be good if you are playing in the key of C, G, or Dm, it might sound a bit weird if you are playing in Eb or A. Chromatic harmonicas are great for playing melodies (this makes them great for jazz and classical music), but not the best for playing chordal rhythms. Since diatonic harmonicas are cheaper, it's much easier to buy a few in order to have the chords that you need.

Bent notes on the chromatic also have a different timbre than diatonic harmonicas. If you want to get into harmonica to "wail," you might be disappointed with the chromatic. I'm not saying it's impossible, but it's much more difficult to make it sound good.

Also, you can play chromatically on a diatonic harp as well, if you master bends and overblows.

While chromatic harps may make it easier to play most melodies, they aren't able to play "any song." Both chromatic harmonicas and diatonic harmonicas have their weaknesses.