This might be a good place to start, it's a long read but pretty interesting.
Note that in some ways, more modern consoles may actually be easier (or at least have different problems) - there are a whole bunch of abstraction layers that stop programmers from doing weird cycle-accurate tricks, and they don't have the development time for it anyway.
Modern may be "easier" because they are being built on top of hardware that is similar to what is found in modern consumer pc systems. That's no to say they are nessessarily the same but being x86 based means they have many of the same instruction sets.
Because afaik emulators, especially of older consoles, have to mimick in software every aspect of the original hardware of the console. This obviously takes a lot more computing power than just what the original hardware needed to run the games. I don't know how this situation has changed, however, as the latest generation of consoles are x86 based.
It's not so much the software that they need to mimic but more so the hardware. They have to emulate the hardware to be able to run the instructions on a different cpu/gpu/audio architecture than what the game was built for. That's why it takes a much more powerful system to emulate a much weaker system.