To the extent this would not be the case, the main reason is likely to be that investors attribute a high probability of government intervention and bail-out in case a failure of a bank due to its low capital. Implicit government guarantees are not a good substitute for well capitalized banks.
They would be distortive, expensive for tax payers and leads to too much risk-taking. It is an investment worth taking even if funding costs and therefore lending costs would increase marginally due to higher capital requirements. Financial crisis are very costly and it has a strong payoff to reduce their frequency.