Norman castles were designed for a different purpose, they were not defensive structures like the burhs, they were designed to intimidate the conquered Anglo-Saxons and remind them of Norman power.
Norman castles were often built in locations that were considered of strategic value. The first Norman castle in England was built a few miles from where William landed and was used as a base for soldiers to terrorise the local population and gather supplies.
Unlike Anglo-Saxon fortified towns, a Norman motte and bailey castle could be built very quickly, in some cases it only took a few days.
William had 8000 men to try and consolidate his power in England. Building motte and bailey castles were an effective way of securing towns that had submitted to his power.
Although the wooden structure was much more vulnerable to damage than a stone structure, a motte and bailey castle could be built quickly until the Normans had the time to build more permanent stone structures.
The major weakness of the motte and bailey castle was the likelihood of the keep rotting or burning down. The solution was to build stone keeps but these could not always be built on the same site since the weight of the stone would sink into the motte.