After listening to ATC communication (source:
http://archive-server.liveatc.net/kb...2018-1200Z.mp3 minutes 10:34, 12:28, 13:50) and corroborating with local weather conditions in BOS (fog, low visibility, low clouds), they diverted to ALB due to low RVR that failed to improve.
The RVR was consistently <1,200ft as reported by ATC, METARS are similar. They diverted because they would be low on fuel if they kept waiting for conditions to improve.