Dual carriageways don't have a hard shoulder and are national speed limits. What's the difference? There is far more technology on a smart motorway than a dual carriageway. And before you ask, I don't patrol them, I patrol the stretch that time forgot, we still get RTCs due to inattentive drivers.
The difference is expectations. What do you expect not to do on a motorway that you are constantly expecting to do on any other class of road? The answer is simple.
Stop.
Ok, so you want to drive from Bristol (hellhole) to Glasgow (Hellhole). How are you going to do it? On A or B class roads or on a motorway? The answer is motorway because you know full well that providing your tank is full, once you get on the motorway, you won't need to stop or, (with a fair wind), drop below 50-ish for the entire journey.
You know for a cast iron fact you won't be able to do that on A/B roads due to junctions, tractors, pedestrian crossings, etc.
So once you've been on the motorway for over half an hour or so, the vast majority of drivers won't be paying quite as much attention to the driving task as they would be on lower class roads. I don't care how good a driver you are, if you're being honest with yourself, you know this to be true.
Thus you are more likely to be caught out on a motorway than on another class of road, because the normal free-flow of the journey has lulled you into a false sense of security. Take away the hard shoulder and that vehicle in front that would normally pull over onto it is now stopped in the nearside lane. The platoon of lorries in front of you might be able to pull out to pass it but each one will pull over that little bit closer to the stopped vehicle until one of them doesn't have enough time to react and then...
Or it could be a driver who has been at the wheel for too long and is dozing. WHACK!
You can't account for human nature but you can mitigate against it, The hard shoulder is mitigation. Lack of hard shoulder on a route where you don't expect to have to stop or come across a vehicle in a live lane isn't.