Beyond supply and demand, even the most precise large format film systems were never designed with the tolerances required by a 5.2 micron digital back.
As you tighten tolerances manufacturing costs (materials, labor skill, time, % of rejects, cost of machines required) go up very fast. Want to make something within 1mm? 0.01mm? High res digital backs need tolerances measured in microns (1 micron = 0.001mm).
And as noted above used large format film cameras are in great supply and weak demand while digital accessories to make them viable digital cameras are in moderate supply and moderate demand.
Doug Peterson