Manufacturing costs and demand.
Originally Posted by Rudeofus
The formulated batch sizes for Ektachrome were too large so they had to throw away barrels of dye and emulsion for each run, and the cost to reformulate the film wouldn't have turned a profit. When new regulations meant that Fujifilm had to reformulate Acros 400 in 120, it was discontinued due to lack of profit due to the cost of reformulation. The demand just wasn't there to support the cost of reformulation, for both Kodak and Fujifilm.