Because thanks to the extreme cautiousness of Bitcoin developers who have abundantly communicated the (small) risk of accepting zero-confirmation transactions to the public, people have generally heeded the advice of waiting for at least 1 confirmation.

But in reality, if it is OK for a shop to tolerate the risk of credit card chargeback fraud (which is real and is exploited every day) then it is OK to also tolerate the risk of accepting a zero-confirmation Bitcoin tx. Notice the language I use. "Tolerate" doesn't mean "annulate", but "accepting some degree of risk that can be controlled."

Of course the downside of this cautiousness around zero-confirmation transactions causes people to think Bitcoin transactions are slow, when it's not true. They are as fast as, if not faster, than CC transactions.