Note: If you're trying to have all your Personal Identifiable Information deleted, please read this guide.
First, make sure you understand the difference between wallets and accounts.
It is not possible to delete accounts. This is because each account you create automatically generates a receiving address that would stay valid even if you were to delete the associated account. Since it is not possible to prevent anybody from sending transactions to the receiving address linked to an account, you would then be at risk of receiving funds you can't access.
By not enabling account deletion, Blockstream Green makes sure you always have access to all addresses created within your wallet.