Why are some mod vehicles not in career mode, and How are some of em not able to be in tha mode

So ive been installin sum modes for tha game, specifically for career mode. However, some of the mods i installed make it to the career mode vehicle marketplace, this got me wondering why and how are some of them not in career mode or take the AI traffic, some of the cars are not mod cars well some are but why and how are SOME cars are not chosen??