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"