That's because SC2 has a single player campaign. That you can't play multiplayer. And you can't earn achievements while offline.
PS: No Diablo game has ever been "single player". You mean LAN mode. Diablo 1 was a LAN game, Diablo 2 had an online version (which everyone played) and a LAN version thrown in for backward compatibility, which had to be completely quarantined from each other due to the LAN version being wide open to hacks. Those hacks ruined D1 and D2's offline component to such an extent that there was no point in even playing them. In D3 they decided they didn't want to support an outdated and hackerific gameplay method like LAN when online was overwhelmingly dominant and the intended method of play.