Why .avi format ?

From reading this newsgroup I understand that .avu format seems to be the
conventional file format for digital imaging. I usually associate that
format with motion pictures. Why is that the format used for
time exposures through a telescope ? I undertstand that .jpg wouldn't
give one access to the data on a per pixel basis, but wouldn't a .bmp file
work ? Is the .avi format chosen only by Toucam users ?

I'm also surprised by the heavy prevelance of Windows software for this.
Are FreeBSD, OSX, Linux, Solaris really as rare as they seem to be within
the Astro imaging crowd ?