As with most sports, you can argue that the athletes have been getting better over the years and decades. They have better form and technique, are more specialized in what they do and tend to stick to a sport from a younger age, have access to better nutrition, and with modern sponsorship they can fully focus on their sports. Based on these factors one expects records to improve over the years, until a human productivity plateau is reached under the rules of a sport, which can cause stagnation of new records, or the rate at which they're set.
There are also other factors that can effect the rate of new records; we'll focus on technology. In a different sport for example, the Fastskin swimming suit resulted in many swimming records being broken. But eventually the technology was banned, considered "technology doping" as it gave advantage to those swimmers who wore them. Change in technology can also lower/decrease the records. For example after Uwe Hohn's javelin throw of over 100m, the javelin was redesigned to be thrown shorter on purpose, as athletes were starting to throw it far enough that it was becoming dangerous for the other events happening on the track.
Technology may have had the greatest impact on Pole Vault when it comes to improvements. You can read about the evolution of the poles here. Poles have changes from stiff wood, to slightly bendable bamboo, to steel and aluminum, to carbon fiber these days. I personally don't follow pole vault enough, but suspect even the fiber carbon technology has had improvements over the years. The jumpers get their energy to gain vertical distance from their run up, so the better the pole can translate the horizontal energy of the athlete running towards the box, to vertical, the higher the athlete can jump. So with poles that more efficiently transfer the energy, we can expect the pole vault records to get better.
Furthermore, as you mentioned, the records plateaued after 1994. This is because Sergey Bubka was the best jumper at the time and pushed the records to the point no other could. He took it to the plateau point where we had to wait untill "better' suited athletes could make improvements.
The record was eventually broken by 2cm, but Bubka remained the most consistent high pole vaulter of all time. Only in summer of 2022, Mondo Duplantis managed to have more 6 meter jumps than Bubka. Having been born to parents who competed in athletics, and with great talent, Duplantis has already been improving world records since he was 20. He's now doing what Bubka did decades ago, pushing the world record higher few cm at a time.
