Emulators of 32bit platforms aren't quite as mature as that of 16bit ones, but I'd hardly call it "not that good"; PS1 and GBA emulation is highly compatible and from what I hear Saturn emulation has more or less reached that point too.
These days far more emulators are open source than they have been in the past. Furthermore, the only reasonably successful PS2 and Gamecube/Wii emulators are both open source and contributed to by several people. The better of the currently active DS emulators is as well. On the other hand, the currently most well regarded Genesis emulators are closed source.
It may really be the case that newer platforms are being emulated more slowly, but part of it is because serious work started on the emulators sooner in the platform's life cycle. But there's no question that emulators of newer platforms are much more work and much more challenging. Not only are platforms exponentially more complex, they're also much more closed and protected - from almost everyone, not just the general public. Back in the day the console manufacturers had to give developers most of the details of the hardware to allow them to effectively do anything on it. Now console manufacturers not only hide this information but protect it behind layers of security. Finally, PCs just aren't increasing in single core performance like they used to, and you need stronger and stronger single core performance for even highly parallel consoles; at the very least, each core must be significantly more powerful than the core you're emulating, it won't suffice to just have many more of them. Unless things end up heavily abstracted this could end up being a very serious problem for emulators.
One interesting snag in all this is that the console makers themselves have incentive to make it possible to emulate platforms so that their next iteration can retain compatibility. Hardware compatibility is going to become a less and less viable solution - we've already been seeing it thrown out this generation. So whatever abstractions console makers implement in their consoles to help make it compatible for next time could end up making things easier for third party emulators. That is, if they can figure out and decrypt enough.