Shimano employ vast amounts of carbon in their fishing and golf divisions, and have used carbon in the shoe soles and wheel rims for a number of years. They know carbon. They researched its use very extensively before deciding on the specs for 7900. They found that they couldn't make a lighter and stronger 7900 crank from carbon than they could make from aluminum. They employed it where it made engineering and financial sense.

Also remember that Shimano is, at heart, a company that specialized in aluminum forging. They have techniques that are very advanced. Simply put, they found a better and cheaper way to make 7900 components out of aluminum, rather than carbon.

I admire them for not chasing fads and basing their designs on engineering instead.