Fertilisers usually contain varying amounts of nitrogen, phosphorous and potassium. Bonemeal has a high phosphorous content, a good all-round fertiliser required for root-growth and setting fruit, and an adequate amount of nitrogen which ensures robust leaf-growth without suppressing blooms.
Even though the plants looked extremely healthy, the fruit would grow to about two inches long then just shrivel and drop off without any sign of infestation or fungal growth. (Squashes can host phytophthorafungus or be attacked by squash vine borers, though pumpkins are more likely victims.)
I assumed that the problem was due to lack of fertiliser and accepted it, not wishing to add an artificial product; not because I was precious about being an organic gardener, I just want to understand what happens when nature is left to its own devices.
Bonemeal contains a high percentage of phosphorous which is a good all-round fertiliser. Potassium (found in bananas) promotes fruit growth :