A motley collection of words, indeed, TRex. They include non-English words (luau, tamale ), abbreviations (staph), 'scientific' terms (angstrom, octal), classical plurals ( termini, gladiola) and many others. One doesn't really need to ask why such words were reclassified.
For myself, I had no idea what luau or tamale meant when I first encountered them - ('common' words that I had missed in a Chi game).
Look no further than the COD definition to see why oldster should not be common - Informal, chiefly Nth American - an older person.
Easy enough to work out what it means if I come across it in a novel, but I would certainly not consider it part of my active vocabulary; oldie, perhaps, or wrinkly would be much more common in UK.
I have no scientific or statistical evidence to support this, it is merely a hypothesis, but I suspect that when we play Chi we are, at least at first, looking for words that are part of our active vocabulary.