Do you want to keep the world vulnerable for a 6 month window?

There exist far more bugs than discovered bugs. By revealing it, I put some people at risk (those who fail to update), and by hiding it I put more people at less risk (everyone, but only if someone else discovers the bug).

It's a tradeoff, but 6 months is a good window for most people to update, while there still not being too much chance of the bug being independently discovered.