Where the nobility live.

[villageyokel]

Why it is that most nobles don't live in the county of their title? e.g. the Duke of Norfolk's family seat is in Sussex, the Earl of Leicester's is in Norfolk?

[kempie]

The Duke of Norfolk is also the Earl of Arundel amongst many other subsidiary titles...

http://en.wikipedia.org/wiki/Duke_of_Norfolk

...and is the third creation of that title.

The current Earl of Leicester is in its seventh creation of that title i.e. this is the seventh 'family' to have received the title. A new creation means the holder of the title is less likely to have any connection with the nominal geographic area of the original seat.

http://en.wikipedia.org/wiki/Earl_of_leicester

[garybfsl]

When William I conquered England he allocated the land to his followers in small parcels. So Knight/Lord A would get 2 parcels of land in Sussex, 1 in Dorset, 1 in Lincolnshire, 2 in Lancashire and so on. This was to ensure that an individual could not establish a land base in one area and rebel. This of course did not stop a number of them joining forces and doing the same thing.

This meant that Knight/Lord A in the 10th century might decide to build his main home/castle in an area he liked. later on he might be awarded a peerage but that would be based a county that was available.

On a related subject you might think that not all counties became peerages. Some of the took the name of the chief city:

Arundel - Sussex
Salisbury - Wiltshire
etc.

They never existed at the same time.