It's just designed with a slightly different set of assumptions.

Instead of water freezing and boiling 100° apart, it's 180° in fahrenheit. That makes it so that they're on the opposite sides of a temperature gauge, and a degree of rotation of the gauge matches a degree of temperature.
Instead of zero being the freezing point of water under specific conditions, it's a brine solution whose temperature will stabilize in a way that's useful for using as a calibration point.

Stripped of its context, it's odd. But it's not irrational, just no longer consensus as the standard, and as such deprecated.