Because Blockstream Green uses multisig, with you having only 1 of the 2 keys, meaning that you can't simply just restore your wallet backup to other wallet software like Electrum. The backup pretty much only works on Blockstream Green. So when Blockstream Green closes down anytime soon for whatever reason(I completely doubt it, but still the possibility is there), you pretty much lose access to your funds completely.
It think it works similarly to a Greenaddress wallet. They will generate a transaction with a pre-signed nLocktime transction so you can use it to recover your funds.
That being said, I would never touch anything like this. This feature essentially turns it into a custodial wallet if they ever refuse to issue you the pre-signed nlocktime transaction and even if they do, they can refuse to sign the transactions that you can and force you to wait the pre-determined time before you can recover your funds.
Anything that comes from a company that runs a business is going to always be centralized and should be avoided. Blockstream is not an exception. In fact this is one of the many reasons why this wallet was removed from bitcoin.org ever since it changed ownership and the code was modified significantly that nobody reviewed it after the change.