In the following question, I understand why  "debido al" is a correct option. However, why is "gracias al" not an "ironic" option here?     

No pude llegar a tiempo ________ tráfico en la ciudad.I couldn't get there on time because of the traffic in the city.
In aonther lesson, it states that "gracias a" can be used ironically:Generally, we use "por culpa de" for something with a "negative result" and "gracias a (que)" for something with a "positive result", but sometimes, when we talk "ironically" we can swap them. For example, we could say:
Gracias a Miguel, hoy no hemos ganado el partido.Thanks to Miguel, we didn't win the match today. (ironic - meaning it's his fault)