Err.................. the weather?
Viz, few pollinating insects at flower time, little sunshine to ripen fruit, too much rain causing what fruit there is to go mushy.
It is not only true of sloes, but of most fruits from apples to strawberries.
It seems to be most crops; I was speaking to a farm worker in the Scottish Borders at the weekend, and he said the apples are about 10% of the usual crop, and they haven't been able to gather in potatoes.
The reason for both is the lengthy spells of rain, a) preventing bees flying and pollinating, and b) farm machinery can't move along soggy ground.