I would say mostly because people in windows are unaware of it. Rsync is a command-line utility that is consistent with the unix philosophy of having lots of small tools preinstalled. The windows philosophy is based around GUI applications that are all downloaded and installed separately. There is not a smooth integration spot where rsync would be obvious or make much sense, and running commands on a windows system is tedious at best.
Also, rsync really shines when its part of a larger application (say for consolidating and parsing logs), or as an automated archival system (implemented easily with a cronjob). Windows simply doesnt have the other tools in its ecosystem to make using rsync actually viable.
Finally, I would say that rsync is just too freaking complicated. Anyone I know who uses it regularly has a pre-set group of flags (mine is -avuz) that generally does what they want, but the man pages / documentation lists dozens of command-line switches, some of them amalgamations of other switches. For example (from the [man page][1]):
-a, –archive: archive mode; equals -rlptgoD (no -H,-A,-X)
It is a quick way of saying you want recursion and want to preserve almost everything (with -H being a notable omission). The only exception to the above equivalence is when –files-from is specified, in which case -r is not implied.
Windows users generally expect, well, windows, and menus, and to have a single app be an all-in-one solution, not just an independent piece of a tool chain.