I would say that là is used for concrete/physical things, such as "that is a rock", or "i am a woman", and should be removed for things like emotions or adjectives, "i am rich" → "tôi giàu" and "she is sorry" → "cô ấy xin lỗi"
là isn't used when bridging the subject with an adjective. For instance, we wouldn't say Tôi là giàu for "I am rich". It would seem that in Vietnamese, sorry is considered an adjective. Thus, Tôi xin lỗi.