The original Bal was held in 1958 and all the debutantes were American.
Now that there are more countries where the rich don't feel the need to hide the fact that they're rich, the attendance has become more global.
There was a hiatus in the bal between the 60s and the early 90s, decades when egalitarianism was the zeitgeist. In the wake of the "greed is good" 80s, the rich emerged and plunged into conspicuous and publicised rich-people activities. 2004 saw the debut of Mikhail Gorbachev's granddaughter.
The UK has seen plenty of attendees, including Amelia Windsor, Kitty Spencer, and Tatiana Mountbatten. There have been several Austrian Hapsburg women in attendance, and many members of other European royal houses and aristocratic families.
Bottom line - there are far fewer Americans attending annually than there were (a small fraction these days compared to the original Bal). And the reason for the Bal's continuing status is that there is no such thing as bad publicity.
Americans have their own shindigs.