What would stop 80s computers from using encryption?

Speed. A computer from the 80s wouldn't be able to encrypt/decrypt web traffic quick enough to be usable.

Now I'm curious to know the actual difference in wall-clock time between decryption on a modern device and on an Apple II processor. Seconds, minutes, hours?

According to people at CryptoAncienne (https://github.com/classilla/cryanc), a 25MHz 68030 needs about 22 seconds of maths to handshake a modern TLS server. During that time, most servers close connection.

So on an 1MHz 6502, I think it'd be minutes just for handshaking.