Why wouldn't they? Asia was passed over altogether in Gods & Kings, yet this is the area of the planet with some of the most developed urban societies outside Europe (and East Asia has been covered as comprehensively as Western Europe), as well as a very well-documented history. They've had two civs just from Scandanavia, after all, and mainland Southeast Asia is a far larger region. Every one of the area's modern countries lays claim to at least one notable historical civilisation; Vietnam hosted several, including the longest-lasting.
Unless they add Malaysia (which is not impossible, to really throw things off), it's also the only one of the mainland Southeast Asian countries (other than Thailand, already in the game) that's of any modern political relevance; Vietnam was geopolitically important during the Cold War, and has one of the world's fastest-growing economies today.
I'm not convinced by that reasoning. There was at least one other militaristic state in the game's original release: Tyre. That's still a CS, but it's a different type of CS. While they haven't mentioned anything about adding new CS types this time around, if a CS was made obsolete it may well be that an existing militaristic CS name was transferred to another CS class to leave the vacancy filled by Sofia. We already know there will be vacancies for Rio and Warsaw at the very least.