When you say "better" you are most likely speaking in terms of damage output or ability to kill specific enemies faster. SO yes sometimes legendaries are better for that than exotics because not all exotics are made for that. Exotics are meant to suit the needs of very specific encounters. for example, the exotic sword razelighter kills grounded tier 2-3 enemies much faster than any other heavy weapon in the game... but use it against a flying target and you will have no such luck. Each exotic has their niche... they arent meant to be the most powerful and do everything better than legendary's do. They all stand out in one specific manner.