Because MS isn't offering Insider Slow at this time if you read the most recent news about Insider. Unless you had the choice between Fast and Slow when you first signed up then you will only get FAST. And unless you already uninstalled Insider, when it says there a new update ready to install, you can't do anything to stop it from updating. So if it hasn't said that an update is ready you better uninstall Insider unless you want WP10. You had a choice of builds and it only said FAST right ? and you still agreed and the first update was GDR2 ? then the next will be WP10