The issue seems to be how the data was collected in the Phase 3 trial:

Clinical trials of most drugs and vaccines supporting FDA approval are mainly conducted in the U.S. and Europe, where clinical trial protocols are well recognized. The phase 3 TIDES trial used for Qdenga’s application was run in several less well-off, dengue-endemic regions in Latin America and Southeast Asia.

However, it should be noted that Takeda's drug, Qdenga, is already approved by the EMA in the EU and a couple other agencies. It is just the FDA that is holding things up in the US.