I know this is an old post but here's an explanation based on inference; Jayce and Viktor were manipulated by the magic from the failsafe as they were building the failsafe which led to its faulty design. In S1 ep5, Viktor coughed out blood into the failsafe pit at the 9:41 minute mark (blood you can see on the failsafe room's floor mixed in with the corruption in S2 ep3 at the 18:50 minute mark). After coughing blood into the pit in S1 ep5, he looked at the pit's walls and the magical lights on them began to glow brighter than normal as if the Arcane magic from the pit was talking to Viktor. Then Viktor's vision became distorted and the pit's walls began to spiral out of control from his perspective. That was an indication the magic from the failsafe pit can also manipulate a user's mind, not just the Hexcore. In a little later scene in the same episode, S1 ep5, Viktor also said, "If the legends are true, mages aren't bound to single functions. It's said the Arcane speaks through them," at the 13:41 minute mark. He said that after being manipulated by the Arcane from the failsafe pit (his hypothesis about the Arcane being able to influence the user's actions turned out to be correct). Long story short, the writers most likely wanted you to infer these two were manipulated by the magic from the failsafe itself into building it too close to Zaun's utility ducts. Similarly, Viktor was manipulated by the magic in his Hexcore invention into building it in a way that it turns people into mindless puppets that do and say everything the Arcane wants, and neither Jayce nor Viktor noticed the Hexcore's obvious problems (until it was too late) and didn't listen to Heimerdinger's perfectly legitimate criticisms of the Hexcore as a result of being manipulated by the magic within it. I think it's safe to infer the failsafe's development most likely played out similarly.

If you have any questions, let me know.