Because starting with an 883 is cheaper. But really most people like the heads especially the 91-03 models. 1200s in this time frame came with a big 67cc Hemi combustion chamber, so it unfortunately had no squish bands. By converting a 883 to a 1200 with 1200 barrels and dished pistons with the 48cc 883 heads gave you 10.5:1 compression and you now have a squish band all the way around the cylinder plus the 883 smaller valves and ports and you have torque monster in the low RPM range. 04 and up 883s I think still share the same heads, however 04 up 1200 heads are awesome. These are the Buell heads basically.

Here is a little write up by Aaron of Hammer Petformance http://www.hammerperf.com/ttxlheads.shtml