Convenience.
- I got tired of dealing with the original NES's 72-pin connector literally decades ago.
- The original Famicom is just completely out of the question. (Hard-wired controllers = nope.)
- The AV Famicom would actually be pretty nice, and if I ever had an interest in JP games, I'd consider an upgrade to that eventually, but if your main (or in my case, only) focus is American games, then it's not that appealing. US cartridges already stick out quite a bit on US top-loaders; chaining them on top of another converter is pretty silly looking and probably less stable than I'd prefer.
I'm pretty sure you can get mods done on a US system and flash cart to allow for extra Famicom sound, but I haven't looked into it too deeply.
And despite the fact that Japanese Mario 2 is one of my favorite games of all time, I've never been that keen on hunting down an actual FDS. Those things just sound like too much work to maintain and most of the notable games got NES conversions in the west anyway. And again, flash carts can play those games.