What a beauty!!! You have a real find there!!! I worked summers in a Honda dealership at that time and those were great looking and running bikes.
Your question seemed to be why there are not more. The people who bought them chosen them over the 750K and 750A and were usually hot rodders who also liked to customize the bike with aftermarket exhaust or paiint and also put alot of hard miles on them. Less likely to be the type that owned yours and carefully cleaned, maintained and garaged it.
Its competition was largely the GS 750 Suzuki, an equally good if not superior bike, and you find very few of those as well.
In any event, great bike, enjoy it!!
john
1976 CB 750F