I don't remember if I've commented in here before, but I do recall seeing this thread on numerous occasions.
I'm willing to bet that there are two reasons as to why this is not feasible to implement. The first reason is that it is substantially more taxing on resources for the website. Every single difficulty of a beatmap that has scores saved to it are going to require a point where all of that data can be held. People need to also understand something else with that: just because you can only see the top 50 on a beatmap's page does not mean that the rest of those scores are not put into database tables. There's a reason why when a song is picked, you see #x out of y players. Sure, there are a lot of beatmaps now, and one could ask "well, if there's this much here, why can't there be more?" There are a finite number of resources for holding all of these scores and other data, and this category increases the flow of maps having data associated with it -- the consequence of this becomes scalability. Resources would be utilized at much higher rates than before.
The second reason I could see this not being implemented: it sets really bad precedents. While I myself do not agree with some aspects of ranking criteria, it does exist for a reason, and it helps to define a very clear scope that content must follow before being ranked. Making this secondary category sets up a situation for tremendous amounts of subjectivity. Until very clear parameters are set for this, it will be nothing but problems -- and at that point, if you're going to set so many requirements up in the first place, you might as well try and achieve ranked status.