Because Mullvad allows for totally anonymous payment, and is thus preferred by the nefarious.

I assume Google only allows you to pay with a payment card as usual, and even getting a Google account requires a phone number now.

I use Mullvad all of the time. Some of their locations' public exit IPs are blocked for some/all services. Can't log in to HN from some of them, can't use Etsy at all from some of them, can't checkout on eBay from some of them, etc.

The same reason why tor exit nodes are blocked/limited by services. You have the anonymity (if your opsec is good) and endless bandwith. So these will very likely be used for shady/illegal stuff. To do this with google or other providers would be not very smart, the majority of people will use these only for hiding your traffic from your isp, your local network or geo location spoofing and the percentage of suspicious activity from their IP ranges will be very small.