There are so many reasons why, but it's pretty symptomatic of modern day football in Britain. It's sanitised, dull, everybody is seated, and it's been this way for so long that it has become the norm to turn up and sit down for 90 minutes without making much noise. Now, if you sing then you get funny looks. It's a weird thing to do and out of the ordinary.
The reason why away fans are louder, the reason why teams like Palace are so vocal, the reason why the atmosphere on the continent is 10x better is all the same - organisation. We have vocal supporters, but they're spread out around the ground and so nothing gets going and spreads. And when something is organised - like an Ultras type thing at Palace - it's often scoffed at by other fans who don't think we need anything like that to make some noise. Well, yes we do. It's exactly what all of these incredible supporters do abroad and it's pretty fucking obvious - grouping all of the most vocal fans in one section and leaving them to do their own thing. When a club announces a 'singing section' then it's also scoffed at. Why? Things are totally different nowadays to the 70s/80s and proper, organised singing areas are a necessity if you want a consistently good atmosphere at a ground (not just for big games).
A standing section would help in that more vocal supporters would naturally gravitate to that area, but it wouldn't automatically make things better. And it's rubbish to say that a few extra away fans means that the atmosphere in the ground would suddenly be much better; I don't remember the atmosphere being that much better a few years ago in the Championship. We're winning matches and outplaying teams, and people said the atmosphere would be better when that happened. Not true!
We need an organised singing area, the name of which does not matter one jot. I'm sorry to all of the traditionalists who will probably scoff at that idea again, but it would sure beat sitting in a library every week. Singing and creating an atmosphere is the most enjoyable part of the match for a lot of people after all!