Why do Gas Laws only use Kelvin Temperature?
First you must know how was Charles' discovered his law?
See on wikipedia Jacques Charles

Here is a quote from Wikipedia


  Around 1787 Charles did an experiment where he filled 5 sites to the same volume with different adds. He then raised the temperature of the sites to 80 °C and noticed that they all increased in volume by the same amount. This experiment was referenced by Gay-Lussac in 1802 when he published a paper on the precise relationship between the volume and temperature of a gas. Charles' law states that under constant pressure, an ideal gas' volume is proportional to its absolute temperature. The volume of a gas at constant pressure increases linearly with the absolute temperature of the gas. The formula he created was $\frac{V_1}{T_1} = \frac{V_2}{T_2}$


As all statements of experimental laws are completely based on the results of experiment performed and cannot be altered. Charles'law is also an experimental law, and it was discovered from the experiment performed by Jacques Charles and he found that the Volume of a ideal (close to Ideal) gas is directly proportional to absolute temperature.

You are saying that


  Why volume can't be proportional to temperature in Celcius?


But how it can be if it is not the result of the experiment.

Suppose a cricketer just scored six and you are asking why can't we write his score as four instead of six. In the same way Charles discovered that Volume is directly proportional to absolute temperature and you are saying why it is not temperature in Celcius.

If you want to know why volume can only be proportional to absolute temperature - experiment is the proof.