I can't speak for every city in the world, but in my own there is a relatively strong Scottish Rite body, and there are a few York Rite brethren but no York Rite bodies formed at the present time. I have a feeling that Scottish Rite is more prevalent in Masonry, but that may not be an accurate big picture so correct me if I'm wrong. But, if it is true for your area, I want to investigate some specific points:
I had a new Master Mason engage in conversation with me about the appendant bodies. I mentioned that I was interested in York Rite (I hadn't yet started, at that time). His initial response felt negative. He seemed to have the impression that it was some weird, deviant form of Masonry, or something. Perhaps he had the impression that it was "unamerican." As we continued to talk he listened to what I had to say, and he realized that both are acceptable Masonic paths. Ironically, our Blue Lodge uses the York Rite version of the Craft Degrees. The ritual format, wording, etc., is entirely consistent between the EA, FC, and MM degrees and the Mark Master degree that I just went through, where Scottish Rite's format is entirely different, and the York Rite in some places referred to as "The American Rite."
Let me clarify that different isn't necessarily an indicator of good or bad. I think both Scottish and York Rite have their places. From what I've read, they teach in different ways and they cover different subjects. Your own particular tastes and interests may lead you to advance in Masonry in one way or the other. Neither should these two routes be seen as the only ways to pursue Masonry further.
So why is Scottish Rite more prevalent in my area?
I can speculate on that. It isn't because its a better rite. It may be because the degrees are easier to put on (Scottish Rite seems to require far less in the way of officers in order to put on their ritual), and it is a faster way to move people through a rite in order to achieve a purpose that has in recent years been fulfilled in another way: To get people into the Shrine Club.
This in itself is not a bad thing. Shrine does much noble work and helps many people. I just ran into an electrical burn victim last night who was helped by the Shrine hospital. Actually, he asked me for a petition for Lodge, which means he has recognized the good work done by Masonry and he wants to contribute back into it, and be in that company.
Shrine has solved the problem of using the Quick-route of Scottish Rite by removing their haut-grade requirements: One only needs to be a Master Mason now (3rd degree) to enter Shrine, instead of a Knights Templar York Rite Mason or 32nd Degree Scottish Rite Mason.
So, historically the problem was complicated: Knights Templar is considered the most "Christian" of the Masonic degrees, and involves the taking of an oath to defend the Christian religion. While Scottish Rite, in its present incarnation, includes Christian symbolism, it puts no such obligation on its candidates. Knights Templar in York Rite, although the last of the sequence of degrees, is not necessarily essential to thoroughly enjoying York Rite. So far, I have not opted to petition for it: I am going to take the degrees of the Royal Arch Chapter, and the Council of Cryptic Masons first, then I will consider and decide if I am going to continue with Knights Templar. At this point, I'm honestly leaning towards a "No" answer on that.
Obviously, for someone wanting to enter the Shrine, especially a non-Christian or someone uncomfortable with that particular obligation, this made a serious choice FOR them. To get to Shrine, Scottish Rite was the acceptable way. Also, Scottish Rite has become quicker, because it has been simplified in the Southern Jurisdiction to only a few required degrees with the others being optional. This means about 4 or 5 additional degrees, which are mostly acted out for you while you watch as an "audience", and you would have passed the requirement and could be admitted into Shrine.
York Rite was more participatory, and had more degrees to go through, with no skipping degrees (except for the Council being optional). At a minimum, I think this would be six degrees beyond the Master Mason degree before you could participate in Shrine.
Scottish Rite, recognizing their place on this ladder, wanted to facilitate the numbers and give something to the people going through it so they have simplified their degrees to make it easier to go through and perhaps in an effort to make the lessons more readily tangible. In the process, I feel that they have removed much of the "meat" from their work, relinquishing it to the textbooks (Morals and Dogma, Clausen's Commentaries, Bridge to Light, etc.)
This has all been resolved in the Shrine Club's decision to make their membership requirement be that of Master Mason only. Now, what do we have left over?
Essentially, the three bodies now stand as options to a Mason, doors that may be unlocked. They will unlock these doors only if they are genuinely interested in what is inside of them (or if they want to attend the banquets associated with them.) The Scottish Rite should be using this as an opportunity to add meat back into their ritual. I hope they do. The people who want to get into Shrine can now do so, which means we should be left with very sincere people joining both Scottish and York Rite, because they want to.
So, I think it is primarily a historical reason that Scottish Rite came out as the "winner" in my city as far as numbers are concerned. There is no reason someone cannot pursue both rites. And if someone has the time, and inclination, it would probably be good for them to do so.
I want to encourage people in areas where one rite dominates to give the other rite a chance. It is well worth the added effort to travel in order to receive these degrees which share in the antiquity of our Masonic heritage.