Trust and credentials
Why do clients leave you?
Short answerMostly acquisition, closure, or bringing IT in house. Occasionally price.
The honest version: businesses get bought and absorbed into the parent company's IT, they close, or they grow to the point where a full time internal IT person makes sense. Once in a while someone finds a cheaper provider. Sometimes they come back.
Any provider who tells you they have never lost a client is either very new or not being straight with you.