ZSNES is an ancient speedhack SNES emulator written in the soon-to-be-deprecated x86 ASM. Its GUI is horrendous, compatibility is lower than any modern emulator, and its accuracy a joke. Yet it is still the most popular SNES emulator.
Someone looked at this when they finished designing it and said, "Yes, this is what I want."
Because of its popularity, many ROM hacks, and ROM hacking tools, were made using ZSNES as a testing platform. However, because of the inaccuracies of the emulator, these tools and hacks are broken on other, more accurate emulators and even real hardware. This is why many people dislike ZSNES, and is one of the major contributing factors of why byuu made bsnes (and, after expressing his views on ZSNES accuracy, why he is disliked by some in the emulating community).
ZSNES was first released in 1997 for DOS and was first abandoned in 2001 after its source was released. Afterwards it was picked up by a small team who started rewritting some of the ASM to C/C++ and porting it to Linux. However, it hasn't recieved any updates since 2007 since it was once again abandoned, this time for good.
So ZSNES has poor accuracy, can't be ported to anything other than a x86, and is resposible for poorly made ROM hacks, then why do people use it? Mostly from people having used it in the past and being too stuborn to try anything else, "It's played all [4] games I've tried, why should I use anything else?" or from people being nostalgic from its GUI (no, really).
The first emulator emulator?
On April Fools 2011 byuu made ZSNES completely obsolete by releasing bZSNES - a modification of his bsnes that includes the optional inaccuracies of ZSNES and an emulated ZSNES UI. This version of bsnes can accurately-inaccurately emulate the ROM hacks made specifically for ZSNES, which means the only thing ZSNES had going for it can now be played on a much more accurate emulator.