AWOL:
Because it isn't implemented.
Actually it is implemented in the AVR gcc toolset. It is just that the sprintf() provided by default in the AVR libc.a
does not have floating point support (to keep things smaller)
and the Arduino IDE does not allow setting the needed linker options to
to use the floating point version.
See my comments in this thread for full details and how to work around it:
http://arduino.cc/forum/index.php/topic,124809.0.html
--- bill