Only SNES, NES, GB, and GBA emulators needed to load ROMs to the RAM. The reason for this was because those cartridges were pretty much as fast as RAM(a lot of textures, sprites etc never got loaded into the console's RAM, they were directly addressed, which is why there was never any loading time for those systems). It's also the reason GBA emulation on DS is so tricky(DS has 4MB of RAM, the ROMs need up to 16 or 32 MB of RAM-speed memory).
This era of ultrafast loading-free Nintendo games ended with the Nintendo DS(handhelds) and the Gamecube(TV consoles). From there on there was no more need to have the WHOLE ROM in ultra-fast access. Emulators put the ROMs into RAM because of 3 reasons:
1. They NEED the speed in order to run. This is only for systems I mentioned earlier, where on the real device the entire ROM was very fast to read.
2. They can get extra speed with the ROM loaded in memory. This is a special case, attributable only to DS emulators. Because while DS ROMs are NOT fast-access, the emulators STILL put them into RAM. They do this because emulating the CPU itself is already a heavy task, and having a memory-to-drive paging system would put further stress on the PC, so even though the original DS doesn't do that, it's still a better idea to put everything somewhere where access is fast. The paging system STILL needs to be emulated though. That is, there will still be 2 types of data in the computer's RAM: the ROM itself and the 4MB of DS RAM. Data will still be moved from the "slower" area(ROM) to the "faster" area(RAM), and there will still be loading screens, even though from an emulation standpoint it's all happening within the computer RAM.
3. It's easier. A paging system is complicated to program, and sometimes it's just best to dump everything in one place and handle it PRETENDING there are 2 different environments.
Again, as far as I know the DS emulator is the ONLY emulator that loads the whole ROM in memory, even though it doesn't need to. In theory you could program a DS emulator which only needs little over 4MB of RAM, representing the console's own and whatever else is needed for the virtualization layer. It was only done so out of ease and is NOT necessary in order to emulate properly. But with today's low-end computers still having memory 4x the largest DS ROM, it was out of convenience. The resource is there, it's fast, why not use it?
Emulators of all other consoles that don't load everything in RAM(GC, Wii, PS2 etc.) will operate the same way as the consoles, only putting what they need into RAM. If a 3DS emulator is made it will most certainly NOT load everything in RAM. At least not now. But let's imagine a future 10 years from now where computers have 32-64GB of RAM. If the old emulators will no longer be compatible and new ones are made from scratch, it's not unlikely that you'll see PS2, GC, Wii, 3DS emulators that load the entire ISO/ROM into the computer's memory. It's just convenient.