I would like to add to the conversation, as a regular user and occasional comment-poster at XDA since the site first opened, don't bother them. As has already been said, many of the developers on the site are simply rebuilding ROMs that were created by other people, specifically the cyanogenmod, AOKP, and Paranoid Android teams. It's not that they're just copying someone else's work though, there's still a lot of tweaking that has to be done, especially with drivers for each device and they do fix bugs. The people who would seem like they'd potentially be helpful for 3DS hacking are the guys who come up with methods to root new devices; the problem is, it's so much easier to root an Android phone than it is to break encryption or whatever is needed for 3DS hacking. Jailbreaking the iPhone was a much more difficult task as iOS was built with a lot more security, whereas I'm pretty sure Google could care less if you want root access on your phone (it's really the manufacturers and carriers who don't want you to have access to the device because you can then remove the bloatware they're paid to install and can get around things like T-Mobile's 3G/4G throttling when you go over your monthly bandwidth limit).
TL;DR: I'm not trying to say the people over on XDA lack many of the skills needed and they are far from incompetent, but they probably do, or the people who would be useful are few and far between.
Now, on to motive for hacking. On Android, the reason people root or unlock their bootloader is to get access to things like custom ROMs (not ROMs like DS game ROMs, but ROMs as in a dump of a system partition containing a modified OS) which can be tweaked to make things faster (via overclocking), run smoother, provide UI customization, among other things. There are also many apps that require root access, including apps like TitaniumBackup that let you backup each of your apps and their saved data individually so that you can always recover them or transfer data to another device, AdFree which blocks browser and in-app banner ads (okay, Google might not like that one), or SetCPU which lets you overclock your CPU. The reason people want the 3DS hacked could be one of two things: homebrew and the ability to play pirat*ahem*... backed up games. The pirate community is definitely the larger of the two. Look at sales data for people who bought an R4 (From this
2010 Joystiq article, over 100,000 R4s had been seized in the UK alone, sales of the R4 cart alone must be in the millions if you consider the smaller DS install base in the UK compared to the rest of the world as a whole and that in order for 100,000 units to be seized, there would probably be demand of close to that many, although Nintendo could be easily exaggerating the number for their own benefit) and compare that to download rankings for homebrew games and utilities, they're not even close. Remember that you don't need to root an Android phone to pirate games and apps, any phone that doesn't block APK installs can do it (almost all of them, save a handful of devices, especially older AT&T units).
TL;DR: XDA contributors are doing what they do for totally different reasons, mainly they do it for the community.
Which bring me to my final point, community. Over at XDA, they're all about helping new people learn how to develop. Every week there's at least one post on the main page about a developer tutorial or tips from one of XDA's users. When someone has an issue with their build, 9.9 times out of 10, someone else from the community helps them. People who create their own custom ROMs often times use and support other people's custom ROMs. Code for every custom ROM is available in some form (usually on GitHub). XDA developer's probably has the highest reputation in the Android development community. Over on GBATemp, it is exactly the opposite. Someone comes along and wants to try to help but lack any skill, they are flamed (granted, the people at XDA looking for help usually have some sort of firm base or knowledge of programming). People who
do know what they are doing and want to help have their motives and techniques questioned are flamed into retreat (two examples,
Shadow LAG's Wii U Hacking and
fail0verflow). GBAtemp's reputation is probably one of the lowest in the console hacking community (along with the entirety of the PS3 hacking community). Don't take that as a personal attack, I think it's more a case of a select handful of disrespectful individuals who ruin it for everyone.
TL;DR: Why would anyone from the XDA community want to help the GBAtemp community when they can't be bothered to help themselves?
Always touching 