I agree with Larry. I also think cost has a factor to play as well. For grills that typically sell for about $150 to $350, it would probably add another $50 to the price.
Also, there could be an issue with the aluminum expanding and contracting with heat more than the ceramic could handle.
"(Steel has the same value as iron.) The coefficient for aluminum is 2.4, twice that of iron or steel. This means that an equal temperature change will produce twice as much change in the length of a bar of aluminum as for a bar of iron. Lead is among the most expansive solid materials, with a coefficient equal to 3.0."