It comes down to personal preference really. I prefer the soft cases mainly because of the (lesser) space needed to access contents while in hotel rooms etc.
You can have it on its base and the lid open against the wall or luggage rack.
With hard cases since they usually open 50-50 split you really need double the space to have the suitcase open and accessible. This is my main reason.
Thats an interesting design tradeoff I see between the 50 50 Clamshell vs 100 0 Open Box - Division of Luggage Space.
In fact googling and reading online has people who love one vs another.
I personally find "Separation" better - although I do agree that when open it takes double space.
But thats also I find it better for "VISUALLY" seeing and packing per se.
I like the "MORE VISIBLE AREA"
as well as
"DIVISION" across 2 shallows vs DIGGING into ONE DEEP BIN (which I find cumbersome for packing/ finding).
The BIN would require 2 layer deep packing while I'd like 2 separate into shallower "bundles".
Also I started using Marie Kondo style folding/ packing in "verticals" rather than Horizontal Stacks. So more visibility.
If they made hard cases with full depth and just a lid for the top, I would be ok that. Or even a very minimal top half with just an easy access pocket for laptops, gadgets, paperwork, etc.
I think there are hard & soft designs with both - maybe not commonly but there are (Amazon US / Shoppee MY) from my search recalls.
The 50-50 split opening just makes it cumbersome.
Most hotel rooms and luggage racks wont have the space for a hard case to be fully open and accessible.
You hate not having enough horizontal space for the Bag.
I hate not having "More Surface Area" to access if it is a Deeper Bin.
Obviously if you travel alone or typically stay at properties with huge rooms like Aman, then its not an issue.
"aman" ?
The other reason why I prefer soft cases are you can usually pack them in a bit more and even squeeze them in to spaces a bit more than hard cases when required.
Might wanna get some compression cubes & pickup on Marie Kondo/ and other "packing/ folding" techniques.
I actually realized that softer ones are worse off and "blow/ stretch/ give" at the seams when clothes are not folded/ packed TIGHT.
When its done TIGHT, the Hard Walls, hold them in "compression" better.