Why was Mariota signed in the first place & hardly used ???

Most Raider fans were happy when Mariota became a Raider. If Derek didn't produce or was hurt we had a good replacement who would also add another problem to a defense - his ability to scramble. We didn't see him until Carr was hurt and they had to use him. He did pretty good and then we saw him no more. Now this pre season we have yet to see him again, using slightly injured excuse. He is a much better QB than Peterman and should be our back up. If all I say is true then why has he not even played a single series? The excuse that he might play bad and diminish his value is bullshit. He's a former Heisman winner and deserves better.