Because:

a. a used Wista bag bellows is rare - I haven't seen one in 18 months;
b. it is also quite expensive, at around 20,000 JPY, if you find it;
c. the standard bellows works well with 75mm and 90mm lenses, if a recessed board is used;
d. lenses 65mm or wider are best used with a bag bellows, but they too would be easier to use with a shallow recessed board;

Like many field cameras, the standards will almost touch each other when the front standard is pushed inside the body, but focusing is impossible. Ideally, you want the front standard riding on the rail, so that it can be focused. Wista does make an accessory bed for 47mm lenses, but that too is rare and expensive.

Kumar