Why does the married women of Downton Abbey have to have breakfast in bed?

They don't have too, they desire to. When they were single it was important that they appear first thing in the morning for breakfast to insure that they were home and safe. A way for parents to check in on the kids. After marriage they do not owe their parents an answer to whether or not they are home and safe, it is their husbands responsibility to be looking after them.

In an upper class extended family with servants and house staff, the spinster is usually at the bottom of the totem pole since she does not contribute to the household's income, nor is she an heir of significance. As a result, the correct etiquette would be similar to that of a long time house guest which is to enjoy the hospitality, but not to consume unnecessary resources that might be in demand by the actual householders (householders, heirs, and their wives and children).

Young daughters are indulged and coddled until it becomes apparent that they are likely not leaving the household to get married. At this point, they are expected to carry some of their own weight and to try their best to not be demanding. The same is expected of unmarried men (older sons or younger siblings to the heir) once they are past their marriageable age.

As for married women, it is understood that they are consuming resources which are rightfully theirs as the higher ups in the female hierarchy. This does not mean that they lay in bed to be fed grapes by servants all day, but they are allowed a more luxurious start of the day in order to accomodate for the fact that they are married to the men of the house and must obviously be consumed with the numerous duties that accompany managing an estate and its accompanying social conventions.

Because my **** has been cooking in my asss all night and they want a blumpkin pie when it's freshest. In the morning.

My guess is it has something to do with fertilization.