573 is a number pattern which appears throughout the DDR series and other Konami games.
The significance comes from Japanese wordplay. Here's how it works: "573" spelled out would be go-nana-san. "Go" is the sound produced when you add yoon to the letter "Ko". Often when counting numbers rapidly, Japanese will truncate the sound of multisyllable words to a single syllable -> "Na". The Kanji character for 3 is a series of three horizontal lines vertically arranged, which looks similar to the Katakana for "Mi". "Ko"-"Na"-"Mi".