When it comes time to spend your paper wallet, you need to load that private key into some kind of device (eg. Computer, phone, table) and sign a transaction. At this stage, you have to worry about the security of that system. Is it compromised? Does it have a hacked/buggy RNG? Will it leak your keys through a side channel? The trezor does the signing securely on the device. The keys never leave it and never need to be loaded onto another device.
With Electrum, you run it on an offline computer or bootable USB and you sign the transaction there. Then you take the transaction to the online computer and broadcast the transaction. The private key never leaves your offline computer.