I assume that you are asking about Prof. David J. Weber who is a long-time scholar of the history of the Southwestern US and of Mexico. If so, Weber offers three main reasons why Americans have forgotten this war.
- It was fought in the West and outside the US. Weber says we tend to focus on the East when we think of US history.
- It was fought right before the Civil War. The Civil War was so much bigger that it dwarfs other events that happened around it in time.
- It does not reflect well on us. This was a war that we fought with little or no justification other than the idea of Manifest Destiny.
I would argue that the last of these is the most important of the reasons.