A WAN link is a point-to-point link that has two end points. Hence, we need two usable IP addresses to configure each endpoint of the WAN link. Recall that in a subnet, the network ID and the broadcast addresses cannot be used, and hence we will need a subnet with four IP addresses for a WAN link. A /30 subnet has four addresses, and hence /30 IP addresses were used to assign IP addresses for WAN links.
When IP addresses started becoming scarce, new ways were devised to save IP addresses. One of these was defined in the RFC 3021 that allowed using /31 IP addresses for WAN links.