Actually, I suspect the reason is pretty simple. They take a lot of work to assemble.
Even so, I find it surprising that, say, the European Space Agency — which routinely funds global Earth observing products such as GLOBCOVER that are complementary to and different from existing ones from US sources — hasn’t initiated a global population mapping effort.
Gridded Global Population Datasets: LandScan and GPW | Global Data Hound // Apr 23, 2009 at 2:49 pm
[...] Updates for 2009-04-14 ← GLOBCOVER: a step towards a “self-aware” Earth Why only two global population data sets? [...]