Why did the browsers reject it? This question is key. The most likely hypothesis is that the browsers rejected it because it was rubbish - either pointless or overblown (or, most likely in the case of the W3C, both).
You don't need browser support, No-Script and AdBlock Plus seem to get on fine without needing to be incorporated into the core code. How about you develop an addon implementation as part of your feasibility study funding and get some concrete results. Oh wait because that would be actual science and would interrupt all the document writing.
Maybe it failed
because any disreputable web site owner could publish a p3p thingy that would say that they would be nice and caring with your personal info, whilst at the same time collecting and selling it on? This, because a little bit of markup doesn't have the same legal meaning as 22 pages of legalese. That's why there's 22 pages of it...
Oh, W3C, will you /ever/ learn?
P3P was a completely retarded waste of time from the outset. There was nothing at all to stop people from having a perfectly safe-looking P3P policy that no browser would flag a warning about, and then actually doing completely different things with user data in reality.
Unless they propose standardising every single business in the world's backend systems and call centres etc on a single platform so that P3P can interface with it and reflect what's actually happening, or, making having an honest P3P policy part of law so that lying about it will carry some penalty, it'll never work. And I really hope neither of those things happen.
Do users care?
As if any proof was need, the proliferation of sites such as Face Book where anyone can tell the world+dog who they are, where they live and what they are up to at all points in the day, web forums that allow the same, etc etc, shows that most people don't actually care about their privacy.
A Better Idea
Not that I'm a huge fan of CC, the actual license is as stupid as any of them, but it's the thought that counts, I guess.
It was ridiculous
P3P was useless because it works in exactly the same way as the Evil Bit (RFC 3514).
What happened was:
* Site owner finds their site gets blocked by IE6 in its default configuration
* Site owner adds an arbitrary P3P header which stops the site being blocked (usually copy-pasted from some other site), to make the "problem" go away
There's no penalty for publishing "wrong" policy in P3P. So it's just an arbitrary technical hurdle: it shows that the website owner knows how to add a HTTP header, and penalises website owners who either are non-technical, or else fully understand and reject this nonsense.
I expect Microsoft realised that IE6 was just blocking people from perfectly valid websites, and so it was another reason for users to switch to Firefox.
Ever looked at Microsoft.com?
Ever looked at the P3P policy at Microsoft.com? It basically says they will steal, sell, and distribute as much of your personal information as they can legally get away with. So, what's the point?