Team17 simply doesn't want to put out the effort into getting stuff out of Steam. To be more precise, they avoid replacing Steamworks with viable options (let alone options that enable cross-multiplayer) unless it's an absolute must. Older Worms titles (unless they already have cross-multiplayer enabled like Armageddon), are simply left in the dust.
Evidence: the Humble Bundle version of Reloaded which simply gutted online multiplayer in exchange for no DRM, and the sad way in which Armageddon breaks compatibility with some scripts.
Post edited January 10, 2017 by Projectsonic