Mon Dec 11, 2017 10:00 pm
There is no technical reason. It is a safety feature for users.
Most wallet work with a BIP39 seed.
Meaning that you can recreate all addresses with only the seed.
Some people used to import private keys to those wallets and did not realize that the imported private key would not be recoverable using the seed and lost their bitcoins.
Because of that most wallets now do not allow you to import the private key but sweep the funds to an address the originated from the seed.
I guess if you download a full node wallet you will be able to import the private key and spend directly to another paper wallet.