I couldn't say WHY exactly, they just chose TM, but I know that it WAS an HM in gen. 3 and lower. Apparently, in generation 4 it wasn't needed as an HM, after all, there is barely any dark caves in Sinnoh. the only bad one is Wayward Cave. HG\SS needs it as a TM in my opinion, but they had to keep it a TM for HG\SS, because trades are possible between the 2 4th gen. games, and TMs can be held. Imagine having the flash HM in Pearl, and a flash TM that you got from SS. WOULD. NOT. WORK! they made it a TM, and it has to STAY a TM. Unfortunately.