https://www.gao.gov/assets/820/818991.pdf
Here is the full GAO report for public consumption. I know this forum probably wants to read the juicy details of an evaluation between a thrust compounded, coaxial rigid rotor aircraft and a tilt rotor, but those arguments aren't what decided this competition. The document is a dense read if you're not versed in the language of model based systems engineering and federal acquisition regulations but it boils down to one key issue:
Boeing/Sikorsky didn't complete their homework. They didn't read the rules of the assignment correctly and didn't do the work correctly. They asked questions for clarification, were given answers consistent with the assignment and then, inexplicably, decided to TURN IN LESS HOMEWORK in a critical area than their first submission. Because their submission lacked the detail to convince the USG that Boeing/Sikorsky understood the interplay of requirements, subsystem functions, and the rest of the elements of the systems engineering model, it was marked as "unacceptable" and they could not be awarded the contract.
Even though there was a significant cost difference between the proposals, without a complete system model, the Army couldn't evaluate if Boeing/Sikorsky's price was realistic or not... and it didn't matter because their submitted proposal was technically unacceptable.
Cutting through the jargon, it's a brutal document and not centered around any of the relative strengths/weaknesses of the aircraft concepts that have been discussed ad nauseam on these pages and others.