I personally think (mind, this is an assumption, not factual) that the decision is made mostly by how abuse is handled and whether they allow subscribers to set their own rDNS.
Vultr might be okay for outbound relaying. DigitalOcean on the other hand is not. They are very bad at handling and stopping spam, thus ending up on a lot of blocklists.
If you are US based, something like MXroute might be worth considering too.