The reason is that the Celsius scale has a fixed offset.

Can you explain more on this? I still don’t get it.

As of now, although I am not a man of authority on this subject, I still think temperature difference can be expressed by using celcius simply because the celcius has the same equivalent difference as Kelvin. The difference of the two value of the same unit will still be the same unit.

First, from here

Since the standardization of the kelvin in the International System of Units, it has subsequently been redefined in terms of the equivalent fixing points on the Kelvin scale, so that a temperature increment of one degree Celsius is the same as an increment of one kelvin, though numerically the scales differ by an exact offset of 273.15.

Secondly from here

The degree Celsius (symbol: °C) can refer to a specific point on the Celsius temperature scale or to a difference or range between two temperatures.