Since the SI unit for temperature is Kelvin, why is the Celsius scale most often used in the science labs?

Question

I don't know that it IS the most often used. For example, almost all of the calculations require changing from C to K (gas law problems, and almost all others EXCEPT those where a difference of T is used). For hands on lab work, however, you are correct that it is the most often used scale. Although I don't know this, I suspect that it dates back to the "old" days when the centigrade scale was the ordinary European system way (for science and for households) of making lab measurement of T and it became more or less a standard out of convenience. Others may know of some specific reason.