Have you got some stats to backup the assertion most don't?-
Renouncing citizenship is a pretty big deal - you have to officially renounce to the UK government and it costs about £400 to do so, so I doubt many people do it. Plus, you can't give it up unless you also hold or are eligible for another country's citizenship because otherwise you won't hold citizenship of any country and won't even be allowed a passport (which wouldn't exactly help if you're trying to move abroad!). Also, unless you plan never to set foot in the UK again for the rest of your life or you must renounce UK citizenship in order to gain citizenship of another country, then what's the point of doing it? If you give up your UK citizenship, you can never again live in the UK without a visa, in fact you cannot even visit without a visa, so why give up your right to return to the UK again in the future?
If you renounce your country then for me you lose the right to vote since you no longer live there and take no part in it's affairs.
If you've cut all ties with the UK and have given up your citizenship, then fair enough, but I really doubt that many people actually to renounce unless they have to.
My aunt has been living in the US for 31 years and she's never renounced her UK citizenship. She didn't gain US citizenship for 23 years (2001) and so she was unable to vote in the US elections for most of the time she was living in the US and was only able to vote in UK elections. If she had wished to renounce citizenship, then she would not have been allowed to do do until 2001, when she became a US citizen too.