The unfortunate truth of this question is that there are still a MASSIVE number of variables that will effect how easy a steel is to sharpen, even eliminating what the OP said.
The main distinct factors are going to be found in the chemical composition of the steel, but that is only in regard to the part that the steel itself plays in sharpening, which is about 50% of the process.
The other 50% of the process is all in the person who is sharpening, their skill, and what they consider "easy" and "sharp". For example, my cousin is not that good at sharpening to a fine edge, but he is very good at working his edges into a nice toothy finish that is still very sharp. For him, steels like S90V aren't really that hard to sharpen, but he has more issues with some of the more simple, low-alloy steels that don't take a toothy edge as readily, even though many other people consider those much easier to sharpen.
Not only that, but what you consider "sharp" is sometimes a little bit of a subjective thing as well. Some people don't think a knife is sharp unless it has a fine and keen edge that is polished and highly finished, but other people (myself included on
some knives) only really need a working edge, because they know that is what is better for their particular cutting needs. That's also why you will hear people often have a very different opinion on how long it takes a blade to get dull, even if their method of sharpening is very similar. One person already thinks their edge is dull while the other is able to keep using it for quite a while longer.
Beyond that is also the inherent skill in sharpening, which is a hard thing to really acquire, but very valuable once you have it, because it will make sharpening almost any steel very easy when you have the right materials to get the job done.
But the steel itself will play, again, about 50% of the role in sharpening, if you're using the right materials to sharpen (I'll assume we are for this since that's a whole other issue by itself), and you already have the needed skills as well.
Grain size will play a role, in that a knife with larger and harder carbides will be more difficult to give a fine edge (i.e. S90V), while a knife with extremely small carbides and a very clean grain structure will take a polished edge much more readily, and will hold a fine cutting edge and smooth apex to that edge much longer in general (i.e. M390).
In addition to that, the wear resistance of the steel is largely determined by the carbide structure, and that is usually going to play a pretty big role in how difficult it is to sharpen as well. Higher hardness in the same steel will usually lead to higher wear resistance, but to varying degrees for different steels, and only given a proper procedure in the heat treat.
Extremely high wear resistance combined with a very fine grain structure can lead to a steel that is relatively easy to sharpen with the right tools and technique, but that still has extremely good edge retention and takes a very keen edge for a very long time. A good example of this is ZDP-189, when properly heat treated to high hardness, like is done by Rockstead in their knives. Though there is a fairly good amount of skill needed to sharpen their knives, if you have the right tools and have not just let your knife completely go dull like a novice, then touching up the edge or lightly sharpening it is very easy, and you will only have to do so VERY rarely, considering the insane level of edge retention their achieve on their steels.
That also addresses another very important part of sharpening, which is "What is your blade like
before you sharpen it?"
Sometimes we forget this question, because it is a VERY important one to keep in mind when you're trying to sharpen your blades. If you have let your blade become a butter knife, or you have to re-profile the edge, then the difficulty of sharpening a high-carbide modern steel with high wear-resistance like M390, S110V, or S125V(God help you if you're trying to reprofile this one...
) exponentially increases, but if you are bringing the sharpness back to a blade that has slightly lost it's edge, or it's "bite" then that task will be much easier, and won't really require much more effort than sharpening a steel of lower wear-resistance/grain refinement, such as 440C (again, assuming that you are going to be using the right tools and the right technique).
The short version of this is that it is way to complicated to attribute to ANY one factor, even though we, as human beings, seem to have this ridiculous need to make everything simple and "black and white" when it comes to things that just aren't. There is never one cause for anything, and there is never really an easy answer when the question is in any way thorough. We just want it to be because it is easier that way and we can make ourselves feel better about not actually knowing what is going on.
If there is a need to frame your question with things like "forgetting/ignoring factors A, B, and C, what causes X?", then the question will not have one answer, almost guaranteed, because you cannot fully eliminate the extra variables in a question like that and scientifically determine only one cause, when all of the different variables are all constantly effecting the result you are questioning.
And that last bit isn't a dig at anyone or me being a jerk, and if it comes off like that I'm sorry. I'm a little tired of all of these questions that want a simple answer (at NO fault of the ones who ask them at ALL), and I'm damn glad it if Friday right now, so I might have gotten on my soap box a little bit there...
Also, I am omitting entirely the existence of Nitrogen-based steels in this comment/discussion, because I don't want to write EVEN MORE. So suffice it to say that Nitrogen-based steels would make this even
more complicated than I already have here...yeah, this was the "short" version...
