PHP vs. Other Languages in Hadoop/MapReduce implementations, and in the Cloud generally
PHP is designed primarily as a language for displaying output to a browser. Most jobs being run on MapReduce/Hadoop clusters have nothing to do with displaying output.
They instead tend to lean much more heavily towards data processing. PHP is not the most commonly supported language for data processing, by far. Thus, it's logical that the most common supported languages for data processing-related technologies don't include PHP.