I’ve read in a few places now that one of the reasons scientists give for not taking part in public engagement work is because, bluntly, it’s not part of their funding and they don’t get recognised for it.
This could be about to change in the UK. The Minister for Science, Lord Drayson, made a speech last week that committed the government to rewarding scientists who engage with the public. The government, he said, took public engagement very seriously.
However, the Times Higher Education supplement wrote much more clearly about his speech. See the article here.