If I kept all the updates listed then the page would be huge! I thought for a while and decided that 4 weeks was enough to show for people to get up to speed with what's new if they've been away, and short enough to keep page size manageable.
Updated 12/10/2008: From 12 October 2008 the Updates list resides on its' own page. With the space now available I've increased the updates list to 8 weeks or so, that should be enough for anybody.