It's probably a combination of factors. The biggest one, in my mind, is how well you formed a burr and how well you completely removed that burr.
Chisel ground blades are generally easy to sharpen, especially one with such a wide flat bevel like the Mora you pictured. Deburring is usually very easy also. Just lay it almost flat and do a couple of strokes. Boom, no more burr.
The shape of the Mora is helpful also. A completely straight blade is the easiest to sharpen. You probably formed a very even complete burr on the whole length of that blade. Blades with curves, like the Benchmade are harder to form a complete burr. The straight razor even appears to have a small amount of curvature from base to tip.
The straight razor should have the sharpest edge, as it has the smallest included angle. 10 degrees per side is a total of 20 degrees inclusive. Which is smaller than the chisel ground blade at 25 degrees inclusive. The benchmade was probably sharpened at 15 to 17 degrees per side, or 30 to 34 degrees inclusive.
So, we wonder why the straight isn't as sharp or sharper than the Mora. I can think of two reasons: Uneven burr formation. Did you sharpen this blade by placing the spine flat on a stone as a guide? If not, you might not have formed a nice even bevel. Your bevel might not be fully apexed in some parts of the blade. Angle control is important. The chisel ground blade has kind of a built in angle guide since the bevel is so wide. Straights have their own angle guide in the form of the spine.
Another possible problem is what you said: The straight has soft steel. If this steel is very ductile (bendable) the burr it forms would be hard to remove. You might have to flip the burr back and forth many times in order to get it to come off. If you have burr left on your straight, it's a little bit like a foil blanket sitting on top of your edge. There's a sharp edge there. But it doesn't feel sharp because there's a blanket on top of it.
It's hard to diagnose these things over the Internet. Hopefully I'm closer to the truth than fantasy.
Good luck.
Brian.