Most likely because the symmetrical shape lends itself to a facet structure which delivers the highest volume of light return and sparkle factor, at least when it is cut to the right proportions
Tthe diamond cutter maintains an exceptional degree of precision when shaping and indexing the facet or it might just be because the round brilliant cut diamond happens to be the most popular diamond shape of all time, and thus the diamond cutters just keep cranking them out and retail jewellery stores keep buying them, and shoving them in your face.
If you don’t think there is any validity to that suggestion, just try to find an exceptional Asscher cut diamond, the majority of diamond cutters who were producing them, changed their wheels over to produce cushion cut diamonds when they became more popular and hardly an Asscher is to be found.
Perhaps the modern round brilliant cut diamond is so popular because it is simply a timeless classic, which became the symbol of the diamond engagement ring when it was popularized via the national advertising campaigns sponsored by De Beers back in the post-war era of the 1940’s when the majority of print advertisements run by De Beers featured round brilliant cut diamonds.
Another reason the round brilliant cut diamond has become the timeless symbol of love and commitment might be due to the introduction of the classic style solitaire back in 1886. The design of the ring has been modified and improved over the years, but the basic premise of the ring remains a six prong crown set with a round brilliant cut diamond, atop a narrow band of gold or platinum.
