The main reason airships were replaced with aircraft was cost and speed. While airships are cheaper to operate (they use less fuel), they are more expensive to build. Overall, it costs about 50 percent more to move a ton of freight a thousand kilometers than it does to move it by aircraft. Modern building materials and avionics make airships safer than their ancestors of half a century ago, and thus still comparable in safety to aircraft, but speed is still a problem. Jet air transports move good over six times as fast as airships. This is a critical factor for moving passengers and much commercial cargo. But for military cargo, an airship that moves along at three thousand kilometers a day is acceptable. But to just build airships for military transportation means fewer would be built and the cost per ton moved would go up even more. Legislatures find this cost factor a deal killer, and that will probably keep airships out of the air transport business, civil or military, for the foreseeable future.
Airships (rigid and non-rigid lighter than air vehicles) are making a minor comeback. The military air transport community is particularly enthusiastic. Smaller radar blimps (costing about $25 million each) are already in the works, and unmanned, tethered, radar blimps have been in use for over a decade. But large, long range, airships, capable of carrying hundred ton loads anywhere on the planet, are less likely. The issue isn't safety. Despite a few spectacular crashes in the first four decades of the 20th century, the nearly 400 airships built and used from 1900 through the early 1960s had a comparable safety record with aircraft. But during the same period, more than 20 times as many transport aircraft were built. There was a reason for that.