Because rhubarb doesn't have much pectin, and often add apple juice which helps the jam to set nicely.

Does freezing fruit destroy pectin?

Powdered pectin can be stored in the freezer from one season to the next. Freezing will destroy the gel-producing qualities of liquid pectin, but liquid pectin will keep for two years in a cool, dry place.

How do you thicken rhubarb jam?

Lemon juice contains pectin, which is a naturally occurring substance found in plants. Lemon juice will help to thicken the jam due to the high pectin content in the fruit.