New releases query

Hi all,

This has flummoxed me quite a bit. Some Linux flavors like Ubuntu, Fedora
and others keep releasing their new versions every six months or so, much
unlike Windows. Ok, let me clear am not a troll here and neither a M$ fan
and am using Outlook Express to post this. But what really is the need for
such frequent releases? Does this prod Linux users to change over to the new
release or new flavor? While the enormous efforts of the open source
community is highly appreciated, the question is why aren't the patches or
upgrades released as it happened with WinXP SP2.