Sources close to Lauren tell us ... LC is swamped with business ventures and just doesn't have time to shoot "The Hills: New Beginnings," which premiers in 2019.

It makes sense why LC can't be bothered with a reboot. Been there, done that.

Lauren's already got several revenue streams -- multiple clothing lines and her own stores -- and her business portfolio is WAY stronger than any of her former costars.

She's DEFINITELY got more going on than Justin "Bobby" Brescia.

We broke the story ... Justin bashed Lauren and Kristin Cavallari as "s****y people" in discussing the new cast for the reboot.

Everyone who watched "The Hills" knows ... LC had tons of confrontations.

But, LC's blowing off Justin's comments ... we're told her decision to pass on the show doesn't have anything to do with beef or lingering problems with Justin or anyone else.