I believe this is because of the widespread use of shield projectors which repel anything moving too fast (faster than a slow knife stroke or hunter seeker). Some other interactions around shields:

Lasers hitting shields basically results in a fusion explosion which would bring down the wrath of the Houses/Imperium.

Shields are not used much on Dune due to shorting out from storms and attracting worms. This results in some use of projectile weapons when the Harkonnens seal the Atreides troops in caves with mortars or the Fremen shoot the noses of off the Emperors spaceships with artillery.

Shielding technology that can be miniaturized to protect a single person. Person-sized shields were tuned to only protect against higher velocity kinetics because if it goes too low it stops allowing air exchange.

Which is a goof since air molecules at room temperature whizz at about 500m/s.

It's probably tuned to momentum, not velocity. (Momentum is really what we talk about when we talk about "speed" colloquially anyways.)

How would it work then though, as the momentum of air draft is still much less than that of a slow striking blade.

I think the problem is that you're trying to apply logic to a plot device.

maybe it's in kg/s

kg*s ?

Don't they have some kind of small scale force field technology that stops anything (eg. bullets) moving above a certain speed? iirc there was a duel where Paul Atreides was consistently a tiny bit slow on his attacks, due to his training in aristocratic duelling where everyone has personal shields, which was interpreted as him deliberately toying with his opponent.

For bonus points, energy weapons weren't really used because if one hit a shield, it would cause an explosion (centered on the energy weapon maybe?) that would take out half a city.

> For bonus points, energy weapons weren't really used because if one hit a shield, it would cause an explosion

This never really made much sense to me. IRRC when an energy weapon hits a shield either the emitter or the shield generator blows up. Since shields are protecting something while the emitter can be anywhere to me the logical outcome seems to be energy weapons everywhere and no shields at all.

It totally did not make sense. Anyone would be able to generate a massive explosion from readily available components. And there's no explanation where that energy would come from.

A small edit that would make this work: energy beam weapons could not penetrate a shield, and just reveals the location of the shooting weapon.

That's desperately trying to tweak the technology so that he gets a society where knife fighters matter. Actually, those knife guys would be vulnerable to ordinary riot-control weapons - tear gas, Mace, flash-bangs, big plastic shields, etc. Or an RPG with a solid warhead to knock someone out of the fight by sheer kinetic energy. Herbert was writing before every combat organization had RPGs.

There is mention that the shields act as molecular filters, protecting against poison gases. And the shockwave energy of an explosion was negated as an excessively fast high/low pressure wave front. Same with flash bangs. Big plastic shields are just another close quarters combat device, and not particularly useful unless in a phalanx.

Mass times velocity equals mass times velocity. If you're hit by a solid round with an RPG rocket behind it, you're going down, no matter how good your forcefield armor is. Momentum is roughly that of an entire NFL offensive line.

The general answer has already been covered - their magical shields protect against projectile weapons. (Though of course there's always the feeling that the use of massive projectiles would overwhelm them.)

That said, and something I don't think anybody else has mentioned in the thread, in the first Dune novel when the Sardaukar land on Arrakis and have a bunch of fights after the betrayal they _do_ use projectile weapons, which has the affect of trapping the Atreides who'd retreated into caves.

It is brought up later that these weapons aren't worth returning, because their use was a one-off. These cannons were considered a "quirky surprise" rather than a useful weapon.