Because we know for a fact the Demos is a fool and not to be trusted. After all large sections of it don't think like us and friends.
Therefore we're always trying to limit it and hedge it round. Votes for prisoners? Oh no! Votes for teenagers? Don't be silly! Votes for women? Damn, we already let that one through!
Here, for example, is this huge fuss going on about the Labour leadership election. Ed Miliband, in his wonkiness, decided he wanted to make it more democrat by letting anyone apply for a vote. At the time this seemed like a good idea. We all love democracy in the abstract, just not how it works. And now, oh dear, we've discovered that all sorts of unsuitable people with unsuitable views are entering the polling booth.
Damn you, Demos and your tricky ways!. Why won't you play nice?
Harriet Harman has said she tried to cancel the election to give her time to rewrite the rules to stop the unsuitable people voting for unsuitable candidates for unsuitable reasons. but the lawyers said she couldn't.
(We all hate lawyers- and how they complicate our lives- but fundamentally they're a damn good thing. Good for democracy, in fact.)