NRT was a mess all day - I barely made it out on UA last night. It snowed hard in the mid-afternoon, so lots of tpac flights diverted to places like NGO, KIX, etc. then there was a backup because the snow turned to rain and the wind kicked up, reducing visibility. When the sun set, it turned back to snow and the problems just compounded. They even extended the normal 11pm curfew to 1am and then again to 2am at NRT.
So, to directly answer your question: it was likely aircraft availability and/or the curfew. But there were plenty of flights canceled among all airlines - not just DL.