Because it wasn’t science. :) keep in mind it was before there was a notion that a temperature scale was part of science, it was part of a tool.
“My thermometer is easier to read and the scale is more likely to line up with what you want to measure”.
It’s kinda like how a CD having 700mb of storage is a product of engineering choices and compatibility with older tape/record formats that usually had less than 80 minutes of audio, and not some fundamental measurement about the world.
The science he did was in making methods of consistently measuring temperature, not the numbers he assigned to those temperatures.