Hey, what's the difference between R and a library for Haskell (or even Python)? It's interesting that a language that is limited to a particular use (statistical analysis) has taken off so much instead of a library for a general purpose language.
You helped answer the questions yourself. The fact that it was built for statistical analysis means that the syntax and overall flow is REALLY easy to understand and manipulate data with. I imagine someone who is familiar with Python would gravitate towards Pandas or another library, but for the BI that I do every day, R is perfect.
People love saying that BI/Data Science is 80% cleaning data (which may or may not be true), but I've found R to be the best for cleaning up 100k+ rows at a time.