Why was anhydrous magnesium carbonate “upsalite” discovered so late?
The paper describing the synthesis of Upsalite has a clear introduction explaining the problem:


  Magnesium is the eighth most abundant element in the earth’s crust and essential to most living species. It can form several structures of hydrated carbonates such as nesquehonite ($\ce{MgCO3·3H2O}$), and lansfordite ($\ce{MgCO3·5H2O}$), a number of basic carbonates such as hydromagnesite ($\ce{4 MgCO3·Mg(OH)2·4 H2O}$), and dypingite ($\ce{4 MgCO3·Mg(OH)2·5 H2O}$), as well as the anhydrous and rarely encountered magnesite (MgCO3) [5]. In contrast to other alkali earth metal carbonates, chemists have found anhydrous magnesium carbonate difficult to produce, particularly at low temperatures. Above 100°C, magnesite (crystalline $\ce{MgCO3}$) can be obtained from $\ce{Mg(HCO3)2}$ solutions by precipitation. However, at lower temperatures, hydrated magnesium carbonates tend to form, giving rise to what has been referred to as “the magnesite problem” [6].Yet, not only chemists have been intrigued by magnesium carbonates. Although abundant in nature, where crystalline forms exist as traces in most geological structures, pure magnesium carbonate is seldom found on its own in larger deposits, a fact that has puzzled geologist for more than a century [7].


As you can see, anhydrous crystalline magnesium carbonate has been synthesized for a long time (and occurs in nature), in the form of a dense phase: magnesite. The new material reported under the name of upsalite is simply an porous amorphous polymorph of magnesite. New polymorphs of inorganic structures are regularly discovered/synthesized by researchers, and reported in the literature. The most striking example may be the family of zeolites, with more than 200 polymorphs of aluminosilicate.