I left this comment at Which Episodes You Must Watch To Catch Up On Agents of SHIELD: "Actors can't be better than their material. Speaking as a writer, I assure you, the first person to blame for a lame character is the writer."
That said, the writers answer to the show runner, and the show runner answers to the producer, and the producer answers to the studio, and the studio answers to the advertisers. So ultimately, blame capitalism.
Uh, that's my sense of humor at work. Really, just know that only the most brilliant actors can make a part better than it is, and even they can't turn the appropriately named Ed Wood into Shakespeare.
Which reminds me that I need to see the Ed Wood movie again.