This is a very valid question and I do not have a firm answer for it. There are a couple scenarios that were at play. Dahlin had just come off the worse season of His Career, the Sabres were at the beginning of another tear down and Dahlin and the team didn’t want to commit long term yet.
I think the most logical explanation is the fact Sam Ventura had only been on the job for slightly over two months and had not established himself as a prominent voice in the front office as He and Adams were getting used to one another.
Ventura was one of the first in the analytics community to realize the value of signing certain players to long term deals rather than bridging them off of their ELC. As Shayna Goldman pointed out, players best seasons tend to be immediately after their ELC, so paying for future performance rather than past performance is a better practice. Jeff Marek was talking about how He did this in Pittsburgh with John Marino.