This would be the only reason I think.
First, getting a hold of a PS3 that can run Linux is fairly hard - It really has to be one of the older versions since Sony patched out the capability to run Linux. I know this because I have one of the older versions and I've been avoiding connecting it to the internet for PS updates in order to preserve the ability to run the Linux environment. I think there's a way around the patch, but like anything of this nature, it would be really hacky. With respect to PoI, the machine might make short work of this since in the show, it can use any audio/video device connected.
Second, programming the Cell is really hard. Its instruction set is not the typical one you would find at any computer. Also, I think most of the programmers for the Cell chip complained about the small buffer space (see https://www.reddit.com/r/explainlikeimfive/comments/34maij/eli5_what_exactly_makes_the_playstation_3_so_hard/cqw3fer for ELI-for-basic-CS-student explanation). So not only did you have to combine the data in order to take advantage of the SIMD advantage, you now had to control what went into the buffer using an instruction set that is not taught at a normal university. So, the machine would need to adapt to basically a new brain. This would be highly unlikely.
Finally, a lot of hardware nowadays can beat the Cell processor and be easier for someone (and as a result, the AI/machine) to program. See this StackOverflow post for an explanation why Cell development 'died': http://stackoverflow.com/questions/1660391/death-of-the-cell-processor. Nowadays, if you want a lot of parallelization, you have two options: GPUs or clusters. GPUs work well with massively parallel problems: think anything with matrices. However, you have to rewrite your code to use the GPU (though not as much as if you targeted the Cell). I mention clusters because we now have frameworks like Hadoop and Spark to basically run a program that utilizes massive amounts of cores. Yes, the parallelization is different, but it sooooo much easier than ripping your hair out trying to learn how to rewrite your program for the Cell. So, if it wasn't for the "infected firmware", they would have been better off grabbing any machine.
So it felt like a cop-out to me - why not just grab the CPUs and GPUs created before Samaritan launched? But then again, this is a television show. It was rather cool to see the PS3 cluster applied here. The only thing I think they goofed on was decreasing the CPU utilization once they sprayed liquid nitrogen (ignoring rate of cooling) on the machines.