Wireguard is still very early stage, and many features that are useful for enterprise Software are not available (or even not possible) with Wireguard.
For example as of today Wireguard only works with static ip addresses. Using the official client it is not possible to assign IP (or pass DNS information) through a DHCP.
There is a dynamic ip project in the works, but it's not yet ready for public consumption from my understanding. https://git.zx2c4.com/wg-dynamic
Many VPN providers (I think even AirVPN was in that list) even say today that Wireguard is too unproven (client and server) in day to day life (still new) and not audited. So VPN providers stick to the proven software for now (and some of the VPNs are simply to lazy to adopt to new tech).