Because the majority of 16 year olds, if they even cared to vote, would vote for all of the wrong reasons.
Not to mention the opportunity for adults to take advantage of this and sway them to vote in a certain direction.
They'd really just be used as tools, preventing them from making informed decisions on their own, not that many are able to do this anyway.
I know I wouldn't want these kids on KMC voting on how a country should be run.
Huge decision......too huge for most 16 years to deal with.
Anata wa wakarimasu ka.....