Why diatonic harmonica is so popular?

There are several answers to that question.

Diatonic and chromatic are different instruments (though in the same family) and deliver very different sounds. I play them both, and choose according to the kind of sound I want, though the type of melodic material obviously plays a part - some things are just easier on chromatic, while others are easier on diatonic.

Both instruments can and do bend notes, but the sound of bends on diatonic is much more exciting to the ear, because each bend is produced by both a blow reed and a draw reed contributing to a more complex sound, while on chromatic a bend is produced by only one reed, which is less exciting.

Diatonics give you three chords that are essential to the key positions most players use, and playing chordally in addition to melodically is one of the big attractions of the diatonic. By using a different key o diatonic, you can get the same chordal setup in any key, which is not true on the chromatic – unless you play chromatics that are tuned todifferent keys, and some players do that.

Also, most players learn three key positions, first (same key as the harp), second (the key five steps above the key of the harp) and third (one step above the harp key, or five steps above the second position key). Then they can play in ay key just by using another key of harmonica. Which is much easier than learning 12 very different key patterns on a chromatic.

Bending the diatonic (for me) may not be all that hard, but bending quickly and accurate note pitch can be hard.