Usually I'm afraid it's because there are more category 1 calls than there are ambulances. They have to ask questions to prioritise based on risk to life eg no breathing, poor breathing, weak heart beat etc. The questions are evidenced based, but not perfect. Every call is an emergency (except of course the hundreds that aren't, but there is no easy way to identify which is which, hence the questions). Ambulances do also have to queue at hospital to handover patients, so sometimes many are out of action. More money & more resources are needed, and less time wasters clogging up the system (obviously not this case).