Because they think kids wont know better?

It's always amusing that socialists think that people can't see through their thinking. Then again, perhaps it's just people who can be bothered. Take Demos, it's a left wing thinktank, though you could be excused for not knowing this if you ever went to the BBC site for news. They think the voting age should be reduced to 16, as ideas go I suppose it's not a bad one, though I suspect that many 16 year olds wont, pretty much as a large minority of the population don't.

BBC.

The voting age should be dropped to 16, according to a major think-tank.

Demos claims that one million people aged 16 and 17 are disenfranchised by "outdated" attitudes, and that the case for change is "stronger than ever".

Demos director Richard Reeves said at 16 young people can work and pay tax, or even "take a bullet" for their country but cannot vote in an election.

"They are denied the political capital and social responsibility of voting," he said.

As the nation prepares to face decisions with "profound" consequences for young adults, those young adults should be given the right to vote, the report said.

Demos's research paper warned that an ageing population and the fact older people are more likely to vote meant older people would form the most powerful voting block in future elections.

Demos's research suggested that if 16 and 17-year-olds could vote, 41% would vote Labour, 30% would vote Tory and 21% would vote Liberal Democrat. 

One of the reasons they use is that you can join the army and take a bullet for your country at 17 yet not vote, another is that you can marry and/or legally have sex at 16. They didn't mention having a cigarette or a drink though as this is a righteous pronouncement and those over 18 activities are proscribed.
The real reason though is at the bottom, they think that the Labour party will get more of the vote if they do and that's the most important thing as far as politicians are concerned. They seem to believe that 13 years of Labour will have influenced the young into voting for a get something for nothing lifestyle, oddly enough a Frenchman summed it up quite well...

French Premier Georges Clemenceau (1841-1929): "Not to be a socialist at twenty is proof of want of heart; to be one at thirty is proof of want of head."

The young don't have a clue as to just how much socialism costs the average man on the street, they usually haven't seen a pay packet with the true cost in deductions written on it. They see the benefits and they don't see the cost. This and this alone is why Demos and Labour would give them the vote.