First of all, I bet that the strength of the concrete builds pretty slowly. They're mentioning a 180 day test, where most modern concrete is considered 'fully cured' at 28 days, and generally you're looking at a few days to a week before it's got enough strength to continue construction. (Note that concrete will continue to cure indefinitely as long as it's kept moist and at a good temperature. E.g., I looked at some concrete from a floating bridge that was 40 years old and it was 3x stronger than it was supposed to be based on the original specs. There's some scatter in those, but 40 years in a cool poist environment didn't hurt.) Also note that there are a lot of interesting additives to concrete that can change the behaviour of modern concrete, to make it easier to place, faster to set, slower to set, pumpable, high strength, high stiffness, or whatever.
Second, Concrete tends to be a pretty local thing. You don't tend to ship the raw materials long distances, since they're heavy and low value. In places where there is volcanic ash, it can be used. But then you get to places like the whole US east of the Rockies, and there's none of that.
Third, we lost the recipe for Roman Cement for the better part of 2 millennia, and only recently rediscovered it, well after the discovery of Portland Cement and it's spread through the world.
[1] http://en.wikipedia.org/wiki/Rosendale_cement
(Bonus factoid: while Rosendale cement is named or the location of the limestone deposits mined to produce it, Portland cement is named for the resemblance of the finished product to Portland stone, http://en.wikipedia.org/wiki/Portland_stone)