The devil is in the heat treat.
If you leave the steel very hard (temper at lower temperatures) it's going to take longer to sharpen and you'll need to be more careful to keep the angle the same on each stroke. Take the same steel and edge geometry temper it at a higher temperature and it'll sharpen quicker - the steel will be softer. It'll need to be sharpened more often though.
So ease of sharpening all comes down to how hard the steel is (heat treatment) and the amount of steel you need to work (secondary bevel width).
The Helle blades have a core of hard steel wrapped with a much softer steel. You're not working very much of the hard steel at any point and the soft steel doesn't take much to work.
When you get into the lower end SS, you get steels that are, for lack of a better word, gummy. Grain size is larger than in a properly treated tool steel and working with them can be frustrating. Properly treated 440C is a passable knife steel, regular 440 (or A or B) is not imo. 420 is junk, and you see lots of knives made out of it.
I know Boker uses a lot of C75 (not a stainless steel) and 440C, but have seen a few where they just list the steel type as "stainless"... not very reassuring.