ELI5 : Take the string AAAA.

A simple Cypher would be to change the letters to the next one in the alphabet and offset by 1 for each letter, the message would encrypt to ABCD.

If you try to compress that, well you can't do it, otherwise you lose required information.

If you were to compress AAAA first, you could represent it as the string 4A. You can then encrypt that to 5B.

Encrypting is about adding entropy to a message. Compressing is about finding common groups and represent them differently so that the size is lower. Compressing an encrypted message is basically useless because you added so much entropy to the message that there are no more recognizable patterns to apply compression to.