Sort of based off of this question: http://pokemondb.net/pokebase/156046/why-cant-ampharos-learn-tail-glow
I mean, Ampharos is the Light Pokemon. And according to Dazzling Gleam's description,
The user damages opposing Pokémon by emitting a powerful flash.
Wouldn't it make sense if Ampharos learned the move? I'm not sure if its power or weaknesses have anything to do with it learning the move (except for Fairies, maybe).
Game Freak just made it like that.
Yeah, its the light pokeumon but GAME FREAK obvisily didnt want that so they were like "Nah, we dont want Ampharos to use Dazziling Gleam" So, game freak just made it like that.
Hope I helped!
PS. I made the quote
ALSO, mega ampharos is also dragon, which explains why it cant learn Dazziling Gleam"