The reason vegetable crops are rotated is because if you grow the same family of crops on the same bit of land there is a build up of pests and diseases e.g potato blight in the solonaceae(potato family),carrot fly in the apiaceae family,white onion rot in lilaceae family.

Also different families have different levels at which their roots extract nutrients from the soil-carrots,parsnips,salsify all take nutrients from quite deep whereas lettuce has shallow roots.

You also have a crop rotation to ensure maximum fertility and that the crop isnt swamped with weeds.So early in a rotation you need a crop that is aggressive and will outcompete weeds and after this you need a crop that will improve the quality of the soil.

so a good roation would be

potatoes(to outcompete weeds)

field beans(to add nitrogen to the soil)

brassica(following a nitrogenating crop because its a greedy feeder)

carrots(used later because it isnt very competitive)

there are two other crops that can be used in a rotation in addition to these.Firstly sunflowers are used in weed control as they have a herbicidal effect and marigolds are used to eradicate nematodes in the soil.A fifth option that can be used to make a rotation workable is to have a fallow period on the allotment where you can use a green manure to prevent erosion and leaching of nutrients.possible green  manures are-

grazing rye

red clover

broad beans

lucerne

goats rue

Often using broad beans as a winter cover crop can result in a year round covering of the allotment with a veg crop at the end.