Most commonly because of response latency, a single follow-up attempt, or no named owner at the moment of arrival. These are operational failures rather than marketing failures, and they are cheaper to fix than to compensate for with more traffic.
Count the drop between plausible enquiries and real conversations — it is usually the largest leak.
Five follow-up attempts is a reasonable standard; one is the common practice.
Shared inboxes without routing mean nobody owns the enquiry.