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
Isn't Green Wallet a 2-of-3 multisig where you have 2 of the keys while they have the other one? You can restore it on another wallet or manually sign your transactions if they go down. Here is their FAQ:
https://greenaddress.it/en/faq.html