Why is 'second' commonly pronounced as 'se kend', not 'sek.ənd'?

You didn't offend! Not at all. I don't think RP is necessarily 'dying', although it is, of course, evolving. There have been social changes in Britain however over the last (say) 50 years. People are much less inclined to ape their 'betters', by which I mean that when people traditionally regarded as 'working class' move to become middle class they are much less inclined to modify their speech to be more RP. It is not so much an aspirational model as it once was.

Forgive me Con but exactly what's Received Pronunciation

The way the Queen or Hugh Grant speaks. In Britain (like most countries) you have lots of different regional accents - Cockney, Liverpool, Welsh, Midlands, West Country etc but you also have a kind of 'standard' way of speaking that is region free. It is sometimes called "BBC English".

Thanks Con

http://www.acronymfinder.com/RP.html