Because the previous mechanic was very tricky in hiding it
This guy recently bought this 2003 F-250 Powerstroke at an auction and immediately noticed something wasn't right with it. His problem was that he couldn't pinpoint the issue and neither could his diesel mechanic. Then one day he was looking under the hood and found a very clever hack. To hide the head gasket issue the previous owner or mechanic modified the coolant reservoir by plugging all the ports with silicone.
So there's another thing to look for when buying a truck. Watch here