In essence this guy sums it up perfectly:
Q: Why is WoL not as good as BW from a UMS perspective?
A: Three reasons.
First, we're always nostalgic for the good old days.
Second, somewhat like Cherry mentioned, the SC2 editor increases the complexity and therefore the time/effort costs of mapmaking. Have you tried looking at it?
Third, the system on Battle.net is poorly designed to create communities that play maps together. The two key problems are discovery and coordination - discovery is the process by which players discover new maps to try or communities in which to participate, and coordination is the process by which players all end up playing the same particular game/match together. These are two "problems" any online system with custom maps has to solve. (For gamemaker-produced content like official melee maps, discovery is less an issue than just coordination - hence no need for an open games list.) As Cherry mentioned, the open games list in prior Battle.net games worked well, and it's precisely because it solved both the coordination and discovery problems well. It allowed players to discover new communities and maps because they always looked at a list with essentially everything being played. That helps both to induct players into entirely new genres (say TD player gets sucked into an RPG just this once…) as well as allowing mapmakers to improve on past maps (Civ Sapphire player notices a Civ Emerald game made by somebody else; wonder if that's better…). The open games list also coordinates players because anything on the list already has to have somebody in the lobby waiting, and the dynamics evolved such that people would try to join an open game before looking to host themselves.
The current system solves each of these problems less effectively than an open games list. First, the system has default rules (previously mandatory rules) where players use a popularity system, and many of them will gravitate to the most popular maps in order to ensure they have a full lobby. EVEN IF eight players in a game of Nexus Wars would rather play something else, any one of them stands to lose his chance to play ANYTHING if he leaves the lobby to start something else. So, many players don't. (Not saying all don't; there are definitely people willing to wait or trying to start channels. But what matters is the aggregate.) Second, because this popularity system leads people to coordinate around particular maps, new maps have a much harder time being popularized. Remember, it's not a zero-sum game; Battle.net could accommodate people playing a lot of different things over time. Instead, the system encourages people to gravitate to a few things. That also hurts the ability of old genres to get shaken up by new versions by different makers. Even if EVERYBODY in a phantom lobby today wanted to try the newer, better version, any one of them leaving risks not getting to play ANY phantom because other people join phantom through a popularity list or a bookmark.
Not only does a popularity-based system (whether the old or the new "spotlight" system) not solve the coordination and discovery problems as well as an open games list, it affects the player base in a way that exacerbates the problem. Essentially, people from prior B.net games who participated in particular niche communities no longer have much incentive to keep playing. Sure, some might switch to melee or other custom maps, but what's important is the loss of some of those players in the aggregate. Further, mapmakers with little hope of somebody, somewhere playing their maps will be discouraged and stop trying. Sure, not all - but some. The loss of these players over time or their conformity to the "popular" maps means A) that everybody who keeps trying to play niche games has an even less likelihood of getting to do so and B) that it's much less likely any fix introduced now will rebuild old communities with any speed.
I don't mean to say that Blizzard has to use an open games list to solve coordination/discovery; maybe it can do something else that does. My main point is to show that the current system does not. Further, I want to make clear that having an open games list at all helps; it's just that behavioral patterns and long-term habits mean that as long as popularity is the default the old communities likely won't be rebuilt. I also want to make clear that even in an open games list popular games will get played more and people won't always get their lobby full; the issue is the delta or difference between systems, and I think it's big.