Their historical traffic records are probably good enough to predict how much traffic will be coming in during the different times of the year and they could ramp up or scale down the number of VMs as needed possibly saving them some money.

Also VPS provisioning time is seconds/minutes instead of hours to where they could redeploy to another provider if they suddenly got the boot from one provider. And via Amazon/Digitalocean-type APIs this reprovisioning-on-failure could be fully automated.