Can I earn achievements that aren't specifically multiplayer in Civ 5?
Solution 1:
Playing with in-game modifications enabled will disable the ability to get achievements. I haven't found anything stating that simply playing multiplayer will prevent you from getting achievements but the achievements in Civilisation 5 are very specifically worded.
As you earned a multiplayer achievement during your play time I would go with you (obviously) having modifications off but the single-player achievements only taking effect in single player.
Update: found something, this is an archived post on the Steam forums complaining about lack of features in Civilisation 5 multiplayer.
9) No achievements except for the one "Last Man Standing' multiplayer achievement. Even if you beat a game with a leader you've never played before you won't get the achievement. All but one are only for single player :-(