I imagine, LG, it's because most people seem to hold the view that if somebody has misbehaved badly enough to finish up in prison then, as part of that punishment, they should forfeit some of the rights and privileges that people who behave themselves are afforded.
But there is a wider issue at stake. The Commons voted decisively to reject the notion that prisoners should have the vote. That, as far as the UK is concerned, should be the end of the matter until and unless the Commons decide otherwise.