I am not yet a medical professional (by far!!) so don't try and read any medical advice from whatever I post.
Type 1 diabetes (henceforth abbreviated T1DM) typically has a pretty sudden onset in terms of symptoms (like over a couple of months) and often people don't think to associate the symptoms with T1DM. Some people also have a much milder experience and may not be concerned at all. Also, some of the symptoms are rather vague and, especially if there's poor communication between doctor and patient, may lead to incorrect conclusions. For example, if the patient complains of fatigue and weight loss, a doctor might lean toward more common illnesses like cancer or depression, especially if the doctor doesn't question much further.
Largely the problem is that we don't really have a good way of screening for T1DM, so it can be pretty difficult if the patient doesn't complain about "classic" T1DM symptoms like increased thirst/urination beforehand. As a result, a very large proportion of T1DM patients are diagnosed when they are hospitalized for DKA. My guess is that she had developed T1DM but was unaware.
I'm sorry for your loss.