This answer applies to secondary-level textbooks.

The importance of events is not the only factor that should be considered according to more recent developments in social studies curriculum theory. For examples drawn from the US, take a look at the strands for the National Curriculum Standards for Social Studies and the C3 Framework (pdf). The former stresses the development of civic competence, that is, the abilities and knowledge required to be an active participant in US democracy. The latter encourages that the coverage of content follow what it calls an inquiry arc: the development of hypotheses, the use of disciplinary concepts, the acquisition of evidence, and the development of conclusions and implementations.

These two approaches to teaching history (and social studies more broadly) notably do not quibble about which social events are most important. Indeed, they make no comment on which events are most important. Instead, they focus on civic competencies and themes of human experience. Arguably this is a good thing because being able to distinguish between Egyptian kingdoms probably is not going to matter too much in the life of the average student who won't go on to be a historian. What will matter to future non-historians are things like critical thinking skills, a better understanding of the human condition, and competence as a global, democratic citizen.

Beyond curriculum standards I think there are yet more criteria that should be included, like justice and inclusion. How do we best include underrepresented voices? How do we keep students motivated and invested with content that reflects their backgrounds and interests?

I don't think any textbook for a survey course (world history, Western Civ., etc.) is going to be able to provide thorough coverage of the 'most important events' and at the same time embody justice, inclusion, and skills development. And maybe it shouldn't. The frameworks produced by the National Council for the Social Studies suggest that we should be willing to perhaps, say, skimp on the details of the Intolerable Acts for the sake of more critical thinking about the American Revolution. We should prize close analysis of some content over the rote coverage of the whole 'canon.'

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Edit: I wanted to add that the perspective embodied in the NCSS's standards is very different than the one that some historians take. Some teachers of history argue that a scholarly approach (covering content thoroughly using the discipline's most current research and theories) is sufficient for skills and perspective development because students will pick up on themes of the human condition and how to think critically as part of simply studying history. The NCSS disagrees and contends that you need to actively teach these things, not just assume that the student is going to make the connections and develop generalizable skills on their own simply by reading about past events.