If you are referring to Yu-Gi-Oh! Millennium World then according to Wikipedia
Viz released the first volume of the Yu-Gi-Oh! manga up to the end of the Monster World arc under its original title. Starting from the last chapter of the seventh Japanese volume, the Duelist Kingdom, Dungeon Dice Monsters, and Battle City arcs are released under the title Yu-Gi-Oh!: Duelist, while the Memory World arc was released as Yu-Gi-Oh! Millennium World.
Source: Yu-Gi-Oh! - Manga (Second Paragraph)
What this means is that the Viz Media release using the original name Yu-Gi-Oh! finishes at Millenium Enemy 10: The Last Die Roll which is Chapter 59 of Volume 7 of the Manga in Japan (according to the chapter list on Wikipedia). Yu-Gi-Oh!: Duelist starts from the next chapter and continues to the end of Volume 31 and Yu-Gi-Oh! Millennium World is the final Arc which starts at Volume 32 and finishes the at the end
As for why? it seems to be a decision by Viz Media. while i don't know for sure i would suspect that it was because in the beginning there was several games portrayed in the manga with Magic & Wizards only appearing a couple of times in the first seven volumes. at the start of the Duelist Kingdom Arc Magic & Wizards returned and became the main game that the series focused on (with the exception of Dungeon Dice Monsters). the Memory World Arc however focused on a Table Top RPG (the precursor to Magic & Wizards)
So the first Viz Media release Yu-Gi-Oh! featured multiple games. after that Yu-Gi-Oh!: Duelist focused mainly on Magic & Wizards while Yu-Gi-Oh! Millennium World focused on a entire different game
Viz Media however has also been re-releasing the series as a 3-in-1 edition as the currently listed volumes of it (as of 23/08/2017) have covers from Yu-Gi-Oh!, Yu-Gi-Oh!: Duelist and Yu-Gi-Oh! Millennium World