Why is Kelvin used in the ideal gas law?
Because of 0 (and negative numbers).
The unit Kelvin has a minimum of 0, which is called absolute zero. This measurement is (currently) impossible to reach, so when doing gas stoichiometry with Kelvin, you never need to worry about multiplying or dividing by 0. Using Celsius, it is possible to have 0 as the temperature, or a negative Celsius temperature, thus producing a negative value, which would not make sense.