why is all of Buddhas past lifes referes to times of kings

The title says it all. Why does all the recollections of past lifes of Buddha revolves around times of kings which were similar to the time the Buddha was living. Why aren't there any mention of times that are similar to the times of what we are living right now ? does this mean this is the 1st time in the sansara that we are living in a world like this ? if it is the 1st time, that we are living in a world like this with so many technological advances, that would mean we are living in sansara which is not repeating, which in turns mean that new things can happen in the sansara. then how can we be even sure about enlightenment. if sansara can change what is to say that some day all beings that achieved nibbana will not again be reborn?