He was not a big star in movies, if he was a big star what was will smith
Just admit you not old enough to remember the era we talking about
It's business 101. Brand awareness. As a musician, the more people that know who you are, the easier it is to sell your product. Especially if your public perception is positive.
Starring in a nationally broadcast sitcom and being a supporting actor in multiple big movies gonna boost the public's awareness of you. Nobody saying he was anywhere close to Will Smith as a movie star. This ain't an all or nothing concept
Far as being a celebrity, LL was easily more famous to the public 1995-1999 than all but a small handful of rappers, even some that outsold him.
Combine that with his comfort in the pop lane and Def Jam putting up big budgets for recording and promo and there's a big part of your answer. Mr. Smith had three Hype Williams videos in major rotation. Those videos probably cost more a piece than KRS entire recording and promo budget for his 1995 album. LL had teenage females who ain't know who Kool G Rap or Rakim was buying his albums.