jazz

Several reasons.The chromatic has a reed for every note in the chromatic scale, so you can play any scale, in any key, without having to bend or overblow. Jazz tends to use several scales in quick succession, and having those notes easily available makes it easier to access those notes.

• Having all notes available without needing to get them with special pitch alteration techniques means that all notes can have a consistent tonal quality, like a piano. You can still create all sorts of different tonal qualities, but you're not forced into it like you are when you bend and overblow on the diatonic. Also, the fact that the chromatic is valved makes the blow notes and draw notes consistent in response in tone, instead of contrasting the way they do on the diatonic. These qualties contribute to the ability to play smooth lines at any tempo.
• Having a reed for every note means that every note will be in tune (as long as the harmonica itself is in tune). Playing bends and overblows in tune is harder than it seems, especially when they have to be notes in a scale and not just expressive devices. And the rapid note velocities that are part of jazz make it even harder to play in-tune overblows and overdraws on the fly.
• Looking at it historically, jazz harmonica got started long before anyone knew that it was truly possible to play the diatonic chromatically, and for a long time even harmonica players looked at the diatonic as an incomplete and old-fashioned instrument not to be taken seriously, compared with the modern, up-to-date chromatic, which has all the notes.

All that said, diatonic players have risen to all these challenges over the last 20 or 30 years. But they remain challenges in comparison with the relative ease of playing the chromatic (yes, I really said that).