Because Cardfight!! Vanguard relies heavily on complex card text, triggers, and precise timing, playing the game entirely in Japanese is incredibly difficult for non-fluent speakers. Remembering the exact skills of thousands of cards is an exhausting task.
The vast number of cards in the game means that translating every single card effect is a massive undertaking. Current patch efforts focus on high-priority cards, with partial translations available for less common cards.
The gold standard for 3DS emulation on PC and Android.