Yeah, it's the chalk dust. It's that simple.

Also different colors on whiteboards is occasionally helpful for clarity, e.g. color-coding an equation to a line in a graph. But that's pretty minor.

we had colored chalk for that purpose in my school for math at least

There is much less selection in colors for chalk. And many of them show very poorly on chalkboards. Hagoromo's green for example, is almost impossible to make out on our chalkboards.

I had some teachers allergic to chalk. They sent students to pound the dusters on the balcony.

>Other than the chalk dust,

You know, the worst part that makes them unbearable and a literal health hazard to use.

The fact that it's still used in many places obviously means it's not unbearable. Besides if you're bothered by the dust you can use the water cleaning methods described in this very thread. It's not perfect, but I would take a chalkboard over a whiteboard anytime.

>Besides if you're bothered by the dust you can use the water cleaning methods

Yes, because I carry large amounts of water around with me for every lecture that I teach. Seriously? The board is covered with dust, the chalk tray is covered with dust and the floor nearby is covered with dust. No one cleans them at my school.

The fix is just to use whiteboards which are just better for everyone who doesn't have a fixation with chalkboards.