Why don’t programmers like nature?
It has too many bugs.
Definition: A software bug is an error, flaw, failure or fault in a computer program or system that causes it to produce an incorrect or unexpected result, or to behave in unintended ways.
Puns are jokes that use words that have different possible meanings or words that sound alike but have different meanings (called homophones).
In order to understand the joke, you need to be aware of both of the meanings that the word(s) can have.