What is the difference between a pilot and a purchase?

A pilot is permission to try. A purchase is a funded commitment, and it is evaluated against completely different criteria.

Pilots are approved on promise, at limited scope, often at a reduced or waived price, and with a small enough budget that fewer approvals are required. Conversion to an enterprise purchase is approved on measured results, at full price, with the integration burden at full scale, competing against everything else asking for the same money.

A pilot with no written definition of what it must demonstrate cannot later be argued to have succeeded. That single omission is the most common reason clinically successful pilots fail to convert.