Short answer: yes, if it’s something you can reasonably afford.

Long answer: Like any other game system it has its share of exclusives, mods, hacks, and other stuff. If that library and those capabilities are something you want and you can afford it, then go for it. If it’s outside your price range and/or you don’t see anything enticing enough for the money then pass. High demand doesn’t necessarily mean it’s something you should get, it just means lots of people want it and the supply isn’t getting any larger.

As far as the why: the 3DS has less viable emulation options than other systems (although they do exist and are always getting better), it’s difficult to emulate correctly without two screens and a touchscreen, it has a lot of games that have not been ported elsewhere, there are tons of cool hardware variants, and it became “collectible” as soon as it was discontinued thanks to immediate reselling/scalping.

For what it’s worth I’ve been into game collecting and retro gaming since about 2005 and I expect the 3DS will only continue to rise in price.