I think if you have the kind of in-ear headphones that don't leak - like my Sennheiser CX550s, then there's no problem with you being in the quiet carriage. However, there are a lot of people out there (the majority?) who have no idea or just don't care that their earphones are leaking. These generally tend to be people using Apple earphones, or other, cheap or no-brand earphones. So it's easier to just say "no earphones" in the quiet carriage than get into an argument over whose earphones are leaking noise.
As a bit of an audiophile, I despise cheap headphones - I want to hear music as the composer/band wrote it, not through some crappy, tinny earphones that strip out all the sound quality. I also hate having to listen to someone else's "music" second hand because of their cheap earphones.
As for other sources of noise, if you have a loud child of any age, you should not be in the quiet carriage. If you're going to hold a conversation louder than a murmur, you should not be in the quiet carriage. I quite like that every time I've walked through the quiet carriage on the SWT services I've used, it's been pretty much silent. Then I walk through the doors at the other end to find someone yelling down their phone.
I second what someone earlier said - I travel FC when I can afford to as people are generally more considerate, although there are of course exceptions - I've had wonderfully quiet journeys in standard, and had inconsiderate people in FC.