A simple answer? Both steel and concrete have gotten stronger over the years.
Slightly more complex answer? Materials haven't really gotten stronger, but the higher grades of steel and concrete have become more economical compared to the past. 36ksi steel used to be the norm, and 50ksi was considered high strength, to be used sparingly or for high profile projects like the original WTC. Now 50ksi steel is normal, and 70 or 80ksi is considered high strength. Note that even 200ksi steel was available decades ago, but it's only for cabling, not beams. It will be nice when the day finally comes when we can use 100 or 150ksi beams.
As for concrete, high strength mixes have been available for a while, but were temperamental to use. Newer mixes are easier to work with, thanks to the recent development of new super plasticizer admixtures that allow self consolidating mixes that flow much better into forms and around reinforcement, without requiring extra labor intensive vibration. Rebar can also be placed closer together so the structure size can be reduced. If it's easier to work with, it's cheaper.
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Without doubt the invention of polycarboxylate-based superplasticizers is one of the most important admixture discoveries in recent years. They offered enormous advantages when compared to existing products, but they can come with extra baggage too, as discussed earlier. Fortunately, during the time they've been marketed they have undergone continuous refinement because they are based on engineered molecules. The refinements include extending working time and more consistent results wherever they are used. They are the admixtures that makes SCC possible.
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http://www.concreteconstruction.net/...and-scc_2.aspx
Perhaps most importantly though, is the booming real estate market covering the height premium.