Because they don't want to make any false promises. If they were to say it works on android they have to qualify that with something like "if all the stars align and your carrier, your handset manufacturer, and any other middlemen can't find a way to stop us".

Like with so much other stuff google makes, this isn't really something google needs to market to consumers. This is the bait they are giving to handset manufacturers to incentivize them to push out an update to the latest version of android.