Because it's more useful to compare wealthier countries with other wealthier countries (not by total size of the economy), for these types of metrics. But yes, I agree it would have been interesting to see India and China for comparison (and in fact, they are listed in the OECD report that was linked to in the article).

Update: I noticed that in the linked to OECD report, only the OECD countries, plus Romania, report the same 22-week threshold. That may be why those countries were picked (for a proper comparison with the same IVs.)