Because it was still a very complex evolution that wasn't easy to resolve.
The airmass which wasn't that cold to begin with is stale. You're missing a fresh injection of cold air.
Besides the general lack of cold air, you have two other major problems.
First, the closed upper level low tracks overhead which is always the kiss of death. You need to be just North of the track of the ULL or you dry slot.
Secondly, the NAM and at times the Euro were way too strong with the energy involving the ULL itself. The Euro was closing it off much sooner, which would have led to a quicker development of the surface low and overall a stronger system.
Kudos to the GFS which has had this solution since last week.