I can think of a few possible reasons
1) the Siri remote doesn’t require the higher power input that USB-C provides.
2) the vast majority of people probably don’t have USB-C inputs available just to charge a remote once in a while whereas they’ve more than likely got USB-A easily accessible (in my house for example all my wall sockets have USB-A inputs).
3) Apple have an excess amount of USB-A cables so as a cost saving exercise they’ve included one with the remote just to use up what they’ve got in storage.