First of all, it's untrue that students of other AIIMS need to clear USMLE and move abroad for higher studies. In my sister's batch 5–6 seniors cleared USMLE from AIIMS Bhopal and moved abroad. However, the number of students going abroad from other AIIMS is certainly less than in AIIMS Delhi. Here's why
However, over the past few years, there is a significant decrease in the number of AIIMS Delhi students moving abroad after MBBS. I think the major reason for this change is that India now provides excellent academics and opportunities in medicine that are on par with global standards. Ultimately it's the individual's choice to clear USSMLE and go abroad or stay in India and do their PG from AIIMS.