There is no reason why MPI (which is a concept, not a software itself!) necessarily is easier to install than Hadoop/Mahout. Indeed, the latter two currently are a mess, in particular because of their Java library chaos. Apache Bigtop tries to make them easier to install, and once you've figured out some basics it's quite ok.
However:
- If your data is small (i.e. it can be processed on a single node), don't install a cluster solution, you pay for the overhead. Hadoop does not make much sense on single hosts. Use Weka, ELKI, RapidMiner, KNIME or whatever.
- If your data is large, you will want to minimize data transfer. And this is where the strength of Hadoop/Mahout lies, minimizing data transfer. A typical message passing API cannot scale the same way for data-heavy operations.
There are some efforts such as Apache Hama that are quite similar to MPI stuff IMHO. It is based on messages, however they are bulk-processed via barrier synchronization. It might also have some message aggregation prior to sending to reduce traffic.