Why was Smallville made when it did? Was it necessary, was there a successful Superman trilogy before?
If anything it's being made due to Arrow's success showing that superhero genre can be successful in TV and DC needing more live action things to compete with popularity of Marvel's cinematic universe.
And I don't know who thought Agents of SHIELD would be as successful as Avengers considering gigantic differences between them like one is a tv show and the other is a high budget movie or one having top Marvel heroes while the other has to mix new characters with D-list comic characters but I think it turned out much better than I expected.