Uruguay falls into a cluster of countries that could be called good “all-around performers.” If you look at the table on p. 10 of the ESI report (http://www.ciesin.columbia.edu/indicators/ESI/), you see they fall into Cluster 5. What distinguishes Cluster 5 from the other clusters, as seen in table 5, is that its indicator scores are more or less evenly distributed. On any category of indicators there is another cluster that does better, but it isn’t lowest in any of them. Because the ESI is simply the average of the twenty indicators, the ESI score tends to be high for countries that do relatively well — or even average — across a large number of indicators. This is visualized in figure 2 of the report, in which you can see that cluster 5 has the most even distribution across the five overall components of environmental sustainability.
Eight of the top-20 ESI countries fall into Cluster 5.
As pointed out in the report on p. 11, another relevant fact about countries in Cluster 5 is that they tend to be more democratic and less corrupt than other developing countries.