Beyond simply speculating, it might be that B&H is trying to shift idle stock. This discount is applicable to 35mm, and 35mm is the very poor seller (and has been for many years) of transparency film -- this is the market that is just not there, never mind what Kodak would have you believe. Pro Packs of Velvia 50, Provia100F and Velvia100F collapsed in the market around 2009 and have never recovered. That is why Velvia 50 / 35mm in Pro Packs has also been discontinued. Lots of wooping and fancy statements here on Photrio, but nobody is spending thousands and thousands to rev the market up. Too late. A market for 35mm? Nope. That ship has sailed long ago.
It is very, very uncommon to see larger formats discounted because there are more users of those formats. Other factors concern the retail pricing of transparency film: for a long time has been and continues to be considered as pricey and not of "good value" to amateurs. Professionals using transparency film can still spend around $2,000 a month for the stuff when they are printing from it (in the days of Ilfochrome Classic, expenditure of $3 to $4,000 a month on key transparency stock was common, even more when Kodachrome was a staple along with Fuji for Ilfochrome printing). If people are only going to project 35mm transparency, it is essentially wasted, as there is so much more potential if people would knuckle down and learn about it. Transparency film has a history in print production and that's why there are those that hang on to this and continue to derive the very best results that are available now as they were before digital took hold.