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I think it likely has to do with respective history. Unix started out on the server and evolved onto the desktop. DOS/Windows started out on the desktop and evolved to the server. Unix filesystems were designed in an environment where the machine was expected to run, run, run. Downtime was expensive and to a great extent unacceptable. Defragmenting the filesystem would have been downtime, and thus unacceptable. Current community culture reflects that tradition.

Windows culture tends to look more to resigning one's self to fragmentation (and viruses for that matter) and then running a tool (defragger, antivirus) to "fix" the problem. When NTFS was designed, Windows users were already used to the routine of regular defrags, and would likely do it whether the filesystem required it or not. So why make fragmentation avoidance a high priority?

Edited 2008-10-19 00:46 UTC

True... it's hard to break a habit of defragging all the time; it feels like something's "wrong" or you're missing something after coming from DOS-based Windows to XP in my experience. Still though, I found that I had to defragment every week, *still*, to keep the performance up. It doesn't slow down near as bad as Win9x, but it does get noticeable.

On Linux, I use XFS primarily due to its efficiency at dealing with large files and the fact that it includes an online defragmenter. Fragmentation does still happen, and I run xfs_fsr occasionally, but it only really effects performance when I'm doing something extreme like using BitTorrent to download large, several-hundred-megabyte (or larger) files.

It has to do with the fact the unix filesystems tend to allocate files on either side of the middle of the volume and not immediately one after the other.

This means that there is space after each file for edits and the appended gets allocated with the file as opposed to a fragment in the next empty space.

either way this article explains far better than I:

http://geekblog.oneandoneis2.org/index.php/2006/08/17/why_doesn_t_l...