The main reason is it is difficult for programmers to adjust to Cell architecture. Many devs specially offer Cell programming jobs on their Employment pages. It is a complicated system 6 SPEs, 1 SPE redundancy, 1 for sys software + separate RSX. Not to mention the proprietary and limited amount of XDR RAM. Lack of anti-aliasing is another thing.
Sometimes having the most superb rig is a bad thing, esp when the competitor's are outselling the "superior" platform. This=poor, reduced quality ports when rushed to get to gold.
Optimization is key, it is just a steep learning curve now.