It's a frelling Sandslash, this thing won't be outspeeding anything. Base Speed of 45 won't be outspeeding anything.
75 Base compared to 100 Base. There's quite a huge difference there. And the accuracy is only a 10% difference. And the Flinch thing...30% compared to a critical hit rate of Higher than normal (Which I believe is around 20%) You can easily overcome the miss with a critical hit and still do more damage than Rock Slide hitting twice. On top of that, Critical Hits go through Defense, while Flinches don't.
And to that, how many times have Pokemon used Stone Edge over Rock Slide. Sure Rock Slide has the accuracy, but when the time needs for a quick OHKO, Rock Slide doesn't pack enough power. And how often do Flinches occur, just about as often as a Critical hit. Walls, sure I can understand Rock Slide, but Stone Edge packs more power and has more pros than cons when comparing the two attacks to each other.