Mintmaster
Veteran
Because it comes from the GT200 architecture. It's a step back from G9x in performance density.Why is that chip so big btw? Die size identical to rv740 (no way it keeps up with that...), transistor count almost the same as g92 while having way less functional units (half the rops and texture units, 3/4 the alus, and no unless GT200 it can't claim wasted transistors for DP).
The groundwork for GT2xx was probably laid while ATI still had R600. They wanted to add features for CUDA (DP, larger register file, maybe more), and felt they had room. When RV670 came out, they probably thought it was a decent improvement over R600 in competitiveness, but this was the best that ATI could optimize and it was still a decent step back from G9x. They probably also expected higher clock speeds.
If GT215 was fabbed at 65nm, it probably would be around 250 mm2. Now consider how big GT200 is compared to G92 and the limited peformance gain you get. Using these two pieces of infomation, it's not surprising that GT215 is a bit below 9600GT performance. Transistor numbers are largely PR, so don't get to caught up in that.