To get a contactless debit (or credit) card, I need proof of ID and proof of address.
Octopus cards were launched in 1997. Oyster cards didn't come to London until 2003.
Contactless debit cards didn't come into play with TfL until 2014, and Oyster cards never really extended beyond transport uses in that time.
In comparison, retail establishments, vending machines, parking meters, car parks, were accepting Octopus cards as early as 2004.
There's a lot to be said for that momentum / inertia, which Oyster cards never had, and the accessibility to absolutely everyone without any requirements beyond having the cash to pay for it (approx £15, with £5 deposit for the card and £10 initial credit)
(It's also worth noting I dumped my Oyster card within a couple of months of contactless cards being accepted with TfL, while I still have my Octopus card as it's vital in HK in a way that Oyster never was in London)