One of the factors influencing edge retention is the inherent wear resistance of the steel, the greater the wear resistance the longer the edge retention (though it is not even close to a 1:1 relation). And yes, in general how you achieve a greater wear resistance makes the steel harder to machine, as generally it is done by the addition hard carbides like Vanadium. There are other methods though like carbide dispersal, and grain refinement in general which can increase wear resistance and lower machinability at the same time, but this is a lower order effect.
However there are lots of other factors which influence edge retention. For example how suitable is the steel to the knife. If the steel doesn't have the necessary durability you will find the edge chipping, and thus not only will you have to do a lot of sharpening, it will have to be very extensive because you have to remove a lot of material to get past the chips. It is the same thing if the steel is too weak. if the edge is getting dented and rippled you will again be sharpening long and hard. A lack of corrosion resistance will also do the same thing. If your edge rusted you will have a lot of grinding until you get back to solid metal.
However lets knock all of that out of the way. Lets assume that the steel is picked so as to well suit the knife and thus the edge suffers no gross damage of any knife neither through cracks, dents or rust. If this is the case then sharpening becomes a breeze. Even when the knife gets that dull that it is down to just a few percent of its optimal cutting ability for light work, a couple of passes on a hone will put it right back to maximal, even a light steeling will do the same thing. Something like the Sharpmaker combines both of these into the one tool. This is true regardless of the steel type and I have worked with some of the hardest and highest alloy steels like CPM-10V at ~ 63 RC.
The main problem then isn't that the steel is inherently hard to sharpen, but something went wrong and you are being forced to remove far more steel than necessary. This is what makes a hard to sharpen knife, poor design or materials. Execution could also be a problem, sloppy grinds, burnt edges, etc. , will all make your first sharpening a chore as you basically have to create a set of even bevels, and again you are back to the problem of having to remove too much metal. Letting a knife get really dull before sharpening does the exact same thing, it leads to misuse of the blade as you have to use more force to compensate for a lack of sharpness and the edges takes a beating on a microscopic scale.
The last factor is raw cutting ability. People measure sharpness by how well something cuts, thus the inherently better a knife can cut, the "sharper" it will be rated at any given level of finish. This is why you will often see comments on the "sharpeness" of the forged bowies. It isn't the inherently greater sharpness per say, but the fact that the edges are in general at least half the angle of production knives and thus there is a huge difference in cutting ability and this amplifies how the sharpness is rated.
There is one other minor factor, if the edge is so obtuse than most jigs and other sharpening devices can't be set at a high enough angle, all they will do is sharpen the shoulders of the bevel and thus people will complain about hard to sharpen edges as you can spent an hour doing nothing but grinding on the top of the bevel, again, the fact of grinding too much metal.
So in short, if you want a knife that sharpens easily, get one with a very high cutting ability, with clean bevels, which has the steel picked so as to give the necessary level of resistance to chipping, denting and corrosion (achieves through the proper material and heat treatment) so that there is no gross damage induced through use.
-Cliff
