First while the control plane would be TCP (since it's low traffic), the data plane would then be UDP-only leading to issues where the data plane would not work even though the control plane did. tinc currently starts the data plane out over TCP and migrates it to UDP if it finds it works, and later migrates it back if it discovers it stops working.
Second, wireguard only supports Ethernet frames, while tinc supports Ethernet frames or IP packets depending on the mode. This is useful for, for example, sending a bunch of IEEE 802.1Q VLAN tagged things over the VPN interface. This use case could be migrated to be VXLANs, but it would require breaking the existing tinc contract with its users.
Third, RSA keys are not supported and that is the primary mechanism used in tinc 1.0, it would be a breaking upgrade for all users or require a long migration time where RSA keys were replaced with wireguard compatible ones but both were still supported while wireguard was not used.