Pretty much what's been said already; DS Rom Hacking is just underdeveloped. The tools are incomplete most of the time, but a good portion are open source so other people can contribute/pick up development. And yeah, one of the biggest hurdles is the lack of a user friendly Map Editor.
There's also a huge and significant Advance-Era Bias. I've asked the question before: "Why don't we have more DS Hacks?", and the answers I got were along the lines of:
"Why would/should we research hacking the DS games when we can do everything the DS games can in Fire Red/Emerald? We can recreate the Skill Split System, we can expand the Pokedex to 800, we can implement every move, every ability, we've even added Fairy! The only thing the DS games could offer us now is Pseudo 3D Graphics."
Of course, that's not word for word, and I just combined basically every answer I got into one, but that's the general stances I heard in the past. There's a lot of complacency in the Rom Hacking scene because Gen 3 has tools for everything, and it's extremely user friendly. Because we have so much documentation, so many tutorials. With all this documentation, all these resources and tutorials, why would people want to go and do DS Hacks?
The same reason we do Gen 2 hacks, and Gen 1 hacks. Because we can, and we want to. I would love to see more development in the Gen 4 hacking scene, not just in making actual hacks, but breaking the engine over our knees like we did with Gen 3. Like we have with Gen 1 and 2. A new frontier to establish ourselves in, new challenges, more unique and diverse things. Watching as the engine becomes the plaything of the fans, as they twist it, manipulate it, and make it suit their needs, and in the end, make something even more magnificent out of it.