Because it was a single event that brought the downfall of zeppelins in this universe; Hindenburg disaster. It was a freak accident involving many unique factors such as: USA refusing to sell helium to Germany, Germany deciding to use Hydrogen instead, there being a static build up in the craft that day, there being rain on that day making the field wet, the people in charge of the lines dropping them on the ground instead of wrapping them around their hands, AND most importantly a spark being fired inside the chamber. Then on top of that the sensational coverage of the event and public becoming too scared to travel in Zeppelins any more. Thus it is very improbable for another parallel universe to have this very significant accident. Therefore, there is a near 100% probability of Zeppelins becoming a viable and safe mass transport medium in those universes.