Indexes do not solve the locality problem (see Non-clustered indexes). Even for in-memory databases, it is non-trivial to minimize cache misses in irregular data structures like B-trees.
Now why MapReduce "might" be a better fit for a problem where data fits into one disk. Consider a program which is embarrassingly parallel. It just reads a tuple and writes a new tuple back to disk. The parallel IO provided by map/reduce can offer a significant benefit in this simple case as well.
