This may seem trivial to some, but I found it interesting: the answer lies in mathematics and theory.
In Cartesian space we typically have three coordinates: x y & z. Each view isolates one coordinate, leaving only two variables. In math, when only two variables exist, then you can isolate one to solve the other. When three are present, then the equation becomes somewhat indeterminate. Consider the standard viewport: typically these viewports show elevation or section – a flat representation of the model. We can assume three viewports for the three combinations created with the three coordinates: XY XZ & YZ.
The final viewport is represents what some might consider the fourth coordinate, that being Time. Typically this fourth viewport is a perspective – perspectives show relativity in space; that is, one is closer than another, or distance as representation of time.