Alderson has ridden a roller coaster since signing with the Giants as the 22nd overall pick in the 2007 draft. In his first full pro season, he led the high Class A California League with a 2.79 ERA as a 19-year-old. In the middle of his second, his velocity started to drop and he went to the Pirates in a trade for Freddie Sanchez. In his third, he posted a distressing 6.03 ERA and got demoted back to high Class A.
There's no consensus on how exactly it happened, but Alderson's stuff has declined. When he starred in 2008, he had a lively 88-92 mph four-seam fastball, a plus curveball and a progressing changeup. Now his fastball dips as low as 83 and rarely cracks 90, and while he throws his secondary pitches for strikes, they're nothing special and don't miss bats. He still has good control but it's not quite as sharp as it once was.
Health isn't a factor, as Alderson hasn't missed a start as a pro. He always has had a complicated delivery that featured some herky-jerkiness and recoil. While the deception helps him against hitters, it's possible that the stress in his delivery has cost him some arm strength. The Giants and Pirates have tried to smooth out his mechanics, and Pittsburgh has tried to add some extension out in front, and working on those changes may have worked against Alderson.
Whatever the reason, Alderson isn't the same pitcher he was in 2008, or even when the Pirates acquired him in mid-2009. Unless he finds a solution, it's difficult to envision him making it to the majors.