If they were capable of sub 3:30 then why didn't they run it as their best event?
The pool of vaulters is much more elite. The bar to be a 6 meter vaulter is literally higher than it is to be a distance runner.
Pole vault is a more athletic and skilled event. It in fact is more elite and tougher to succeed than distance running. You need those qualities just to try the event. Most people don't have it.
No knock on pole vaulters who do a very tough event, but the talent pool is necessarily shallow because of the barriers to starting the event, so that very few people ever try the event, in comparison to the sprints or distances. Worldwide, the barriers to trying the 1500m are minimal. You can get your start barefooted, like Zola Budd. There are tens of millions of runners, maybe hundreds of millinos. There are probably no more than 5-10,000 pole vaulters in the world.