Well it probably has to do with something about the recent news about the gpu's quick return bonus.
In there it said some stuff about that before you had to make WU's that could only run on UNI, another with smp, and then the gpu. Now we already have WU's able to work on both UNI and SMP and here soon, GPU's will be able to run the same WU's as SMP/UNI and vice-versa.
If i had to guess why the PS3 is being let go, it's due too:
1. What was once the strong point of the PS3 of having (at the time) nearly the speed of the gpu and the flexibility of the cpu is now being eliminated by gpu's and multi cores/processor computers that can do the same work.
2. the PS3 been a static area in terms of performance over the years. (cpu not getting faster in them and amount of users been flat-lining lately)
3. PS3 probably need a special WU made for it so it cant take these "one WU for all clients" approach.
Basicly to simply put, for what there able to get out of it and the amount of work needed to keep the PS3 going is not worth the rewards anymore.