Because of the risk of disease Dab. Potatoes are prone to clubfoot and blight and trust me you dont want that!! As you want be able to use that patch of your garden for years to come

Several reasons:

Possibility of blight spores or other diseases of the solarnum family in the soil.

It is better for your soil to rotate your crops:
Potatoes and others of the solarnum family require manure/compost in the trenches so you grow those in a bed during year 1.
Year 2 you grow your legumes which do not require heavy manure and fix nitrogen in the soil.
Year 3 you grow brassicas which require a lot of manure compost.
Year 4 you grow your onions and other root veg.

Then if, for example, you have club root in your brassicas one year it is going to be four years before they are back there giving time for the fungus to dissipate.

Basically all four different groups of vegetables require different things from the soil and put back different things into the soil. If you are working your land organically/biodynamically this is vital as you do not use artificial fertilisers. Which do not feed the soil simply keep that particular plant growing - in fact they impoverish the soil over time.

thanks for the explanation. i remember my dad rotating them years ago but couldnt remember why. dab