The main thing that prevented it from going into base is the number of subtleties about what precisely it means to properly "split" something.
Most languages make fairly arbitrary calls on topics such as:
* Do you split on list elements (e.g. ',') or list of elements, so you can multi-character delimiters ", "? What about multiple types of thing that are all delimiters, e.g. any whitespace character?
* What do you do with the delimiters?
* What happens with runs of delimiters?
* What about initial or final runs of delimiters (e.g. leading spaces)?
The end result was that a split package was written by Brent Yorgey back in 2008 or so that rather comprehensively covers the design space, and it was incorporated into the Haskell Platform.