Because Google is expert at making open source stuff become increasingly out of touch with their mainline, closed source releases, while remaining nominally open-source.
AOSP itself used to be 80% open-source Android, and 20% Google's proprietary shit on top of it made it Google's Android. Then slowly, over the years, Google quietly moved the open source stuff into their proprietary stack - or rather, they let the open source versions become stale and innovated only in their proprietary versions. As a result, AOSP is a shadow of its former self, and on the verge of becoming irrelevant in a world where apps all but require Google's stack to function.
They did that totally on purpose: the early days of open Android ensured its success, and now that the market is solidly entrenched, they're closing it and pushing the open players out.
The same will happen with this ChromiumOS-AOSP frankenmonster (I'm assuming they meant ChromiumOS will be merged into AOSP, not ChromeOS, obviously...): it'll be nominally open-source but practically unusable, or at least undesriable to use. And since both will be merged, Google will be able to deprecate both at the same time with no escape path for either of the former halves for the open-source players.
Google never does anything that doesn't serve their best interests. And at this point, usable, open AOSP or ChromiumOS ecosystems are not in their best interests.
n old boarding pass. Maybe you be more specific in your requirements?