What's true main story of "Yu-Gi-Oh" series before it was about TCG? I have read the original of Yu-Gi-Oh series ( The First Generation ). It surprised me that the first chapter ( about 1 to 10 chapters of Yu-Gi-Oh ) it's not about a TCG (Trading Card Game) which people nowadays familiar with. Formerly, the main story is about Yugi and his penalty game, beating some bad guy, compare it with GX, 5D, Zexal, and Arc V which are already about TCG and Duel from the very first chapter.

Does Yu-Gi-Oh actually not a series about TCG from the first ? Why did they change the concept of story all of sudden ? if it's not,  Why the first chapter of the story is not about TCG ?
In the initial stages of the manga, Yu-Gi-Oh was more or less supposed to be a horror manga. Although the end result was a manga about games, it was clear that some horror elements influenced certain aspects of the story. It was later decided that "battle" be the primary theme. Since there were a lot of "Fighting/Combat" based manga, it was in a way difficult to come up with something original. Hence it was decided to create a fighting based manga where the main character does not hit anybody.

Hence, the early chapters of Yu-Gi-Oh feature a variety of different games; but from chapter 60 (volume 7) onwards, the most common game that appeared as a plot device was the Duel Monsters card game (formerly known as Magic & Wizards) through the Duelist Kingdom and Battle City tournament arcs; receiving elevated plot relevance in the latter arc. 

Other games still appear during the DDD and Memory World portions of the manga and gaming, in general, is often referred to; the modern card game being a recent fad in Japan imported from the United States within the original story.

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