Same reason as there is next to no fruit on our plum trees. Wrong weather when the blossom was out. It was so bad here that the bees did not stand a chance of pollinating the flowers.
Strangely though, there are more Greengages on the hedgerow tree near us than ever before.

Some years fruit trees take a rest . . . we have no fruit on our greengage tree this year either. I suspect that Owdboggy's explanation is right.

yes I agree with Owdb there was a cold spell when plum blossom was out. so lack of pollinating insects.