Because unlike most other programming languages, Rust is well-engineered and seeks to both provide the most general abstractions possible and to make the code generated by them as efficient as possible.

In particular, for this task, this requires to:

1. Return references to subranges of the original string, rather than copying them, so that no copy happens if you only need to examine the component instead of storing it

2. Not use reference counting to do so, but rather statically checked references with lifetimes, to avoid unnecessary instructions to update the reference count and lack of a static finalization point

3. Provide a way to get components one by one, so that if you only need e.g. the first two, time is not wasted to split the whole string

4. Provide that through a generic Iterator trait, so that it may be passed to generic methods (like one that collects the result into a vector)

5. Dispatch that generic trait statically rather than using an indirect call as that would destroy performance

6. Make the state manipulated by such an interface into a first-class object, and allow to put them in a data structure (like an array) while still doing static dispatch, so that you can, for instance, split multiple strings into components and interleave them without ever making an indirect call.

The combination of these essential requirements results in the creation of the SplitWhitespace<'a> data type, which represents the state of a parser splitting a string into 'a-lifetime references to whitespace-separated its components one by one, implementing the Iterator trait, and usable in a data structure.