As far as I know it's the "free" factor knowing that you don't really need a hardware wallet of some sort, knowing that all you need is a computer and a printer; and the somewhat false(or sort of misleading) sense of security a lot of guides and articles give their users. That just because paper wallets are offline(but not necessarily unleaked), it automatically means it's "secure".
I always find it strange that people call them "paper wallets", but don't say "USB stick wallets" or "Blu-ray wallets" or "SD card wallets". Putting accent on "paper" just shifts attention from the most important part - how the private keys are generated, and how they are used to make transactions. I feel like some people think that if they don't store their private keys on hard drive, they already have an offline wallet, and this is a really dangerous thing to think.