None of that is relevant here - every effect you list (well, not the first one) tends to increase the perceived variety of Chinese surnames, while the observation we're explaining is a lack of variety. That lack of observed variety is due to an actual lack of variety which your effects have failed to mask. And that actual lack of variety is due to the age of the system.
If your point is the age: most of these names are ancient and can be traced to earlier that the first millenium BC. This is long enough that names can actually start to die out. Also, family names carry great significance in East Asian cultures*. They can carry great prestige, but also infamy. People often changed family names to become less associated with disgraced people. Emperors awarded their surname to loyal and meritous commoners, and these in turn gave it to their followers. Sometimes, whole populations adopted them. This happened to the Li (李), Chen (陳) and Wang (王) surnames.
It's true though that there are actually not that many to begin with. It's just a quite restricted set of words, and because of the writing system there is no variety because of spelling differences. Most surnames are only one character long, and the really long ones are mostly transliteration of non-Han surnames. Also, many non-Han populations were assigned a common surname when they became sinicized.
*: Western family names are mostly rooted in patronymics, place names, professions or adjectives.
