The main reason is the difference in the trucks' gross vehicle weight ratings (GVWR). The C30 is a heavier-duty truck than the C20, meaning its transmission is built to handle significantly more torque and stress. While some components *might* be similar, the overall gearing, case size, and potentially even the bolt patterns are likely different. Forcing a C30 transmission into a C20 could lead to mechanical failure or damage to the truck.
You need a transmission specifically designed for a C20 from a similar year. Always check the specific transmission codes from both trucks for a definitive answer.
