We do know that Katie Cassidy was chosen by the CW as the female lead, not really by the producers.
We have some very strong evidences that Marc didn't get along too well with her.
They were pretty clear that the death on S4 was going to be permanent.
They said they didn't have any stories left to tell with her character.
They brought Black Siren back only to remind us that Laurel was gone for real and to introduce the new Black Canary (I mean, the episode ends with her being sent away and a new tough girl with a canary cry being introduced)
After her death on S4, the ratings dropped a good amount (I don't believe it was only because of her death that ppl turned away, but it kind was the last drop to make ppl leave)
I'm not watching S5, but from what I read, Arrow's been back to being good, but even so, the ratings haven't recovered yet
Black Siren got a good reception from what I read online
That's why, in my opinion, some higher up told them to bring her back, because I don't believe at all that Marc (who didn't even bother to tell Katie that Sara Lance was being introduced on the show as the Canary, didn't really bother to tell Katie in advance that Laurel was in the grave and stated a lot of times that her character was dead for good) would have a sudden change of heart. But again, that's why I think, and even if it's true, we know they will say something like "it was the plan all along".
PS but I'm happy with this
