Many ambassadors are hesitant to request an evacuation of mission personnel. In some cases, they are concerned about the signal it sends to the host government. There are also legal and contractual ramifications related to US companies doing business in the country. An evacuation could trigger a withdrawal of private contractors affecting their businesses. In Iran in 1979, Amb Sullivan objected to an evacuation of mission personnel and was overruled by Washington.
In other cases, Ambassadors see an evacuation as a personal reflection on them and their relationship with the host government. Many times, the host government tries to weigh-in against an evacuation because of the domestic impact. The bottom line is that there are many factors and considerations that go into the decision and it is not taken lightly.
Yes, you're absolutely correct. Don't recall if Indonesia 2002 was mandatory or voluntary (I think the latter), but many left. Even that had nasty repercussions on the business climate.
It does send a difficult signal to the locals and the host gov, and foments all sorts of rumors among everyone (e.g., "Here come the Marines.")
Amb. Martin didn't evac Saigon until the very last moment.