I actually think this has more to do with the textbook authors not thinking of those time periods as "history" but as "current events" somehow and hence not fee

I actually think this has more to do with the textbook authors not thinking of those time periods as "history" but as "current events" somehow and hence not feeling like a "history" book needs to include them. If you were in school about 15-20 years ago, and the textbooks were about 15 years old (pretty common), and the authors were in their 40s or 50s when writing them, then events of 60-65 years ago, so the late 40s and early 50s, would have corresponded to their teenage years... Once you get into the 70s or 80s, it's not just the textbook authors who think that way but also most of the teachers.