Privacy. Google Analytics relies on a cookie controlled by Google, and is likely illegal under the European Union's GDPR. You are supposed to get consent from users before setting a third party cookie, which would require you to throw up a splash screen asking for consent (and just generally annoying visitors to your site).
Plausible Analytics' system is fast, won't slow down your site, and is an open-source company. As a bonus, Plausible also independent, self-funded, bootstrapped and debt-free. In other words, they are independent creators just like Oupost clients.
You can learn more about Plausible on their website. Just as a note, Outpost's group buying power means you get to use Plausible Analytics at a fraction of the cost you would pay on your own.