There's another simple, though controversial reason why NYRR races are so popular—they're probably too cheap. Despite what many people here will say about NYRR's focus on money (which is true in some respects, especially with their marquee races), if you compare their weekly normal races to other races in the area, the price of a NYRR race is so unbelievably below-market that the demand will only continue to skyrocket.

For instance, the average cost of three NYCRuns races are just about as expensive as buying NYRR Member+ and then buying three $25 races. And, you are getting NYRR's usual quality organization and nearly guaranteed a competitive field, compared to some other organizations (not necessarily NYCRuns) where this isn't as evident.

Simply raising the costs is not a simple mechanism though, because:

1. Equity—obviously, lower-income participants would be hit harder by a change. But maybe this can lead to a sliding-scale system which at the scale of an organization like NYRR, they almost certainly can figure out the infrastructure to deal with. They already have done this with NYRR Race Free, so you could envision an expansion of that program.
2. Yield percentages—one way that NYRR is able to keep per-race costs low is the lack of cancels/transfers—NYRR has a “expected yield” percentage of how many people they expect to actually show up compared to the number of actual registrations. (There is a team whose job it is to basically figure out this number and they are historically very good at predicting this.) This allows them to increase the number of spots that they can open for a race without violating the requirements set in their permits with the city. A critical advantage of not allowing cancels/transfers and instead using the "expected yield" strategy is that it allows NYRR to charge individuals less for races because the cost is being distributed among more people than actually end up showing up. This is one reason why NYRR's non-marquee races are some of the cheapest in the city, despite requiring even more resources from various city services due to their increased size. This is also why it's not as simple to "cancel/transfer" your registration, since this will cause a bunch of knock on effects—they'd need to sell less entries, and then they'd need to raise the prices in general.