Because Ваня is a masculine name. It's a diminutive form of "Иван".
Потому что Ваня это имя мужского рода:) Спасибо, если поняли это сообщение. Я специально пишу на двух языках Because Vanya is a masculine name:) I spicially write on two languages
I wrote Ivan and the answer was wrong, but why if Vanya come from Ivan
Vanya come from Ivan
As well as "Ванечка", "Ванюша", "Ванёк", "Ванька", "Иванушка"... You see, Russian grammar allows to create a lot different pet variants of the same name, but that doesn't mean Duo has to include them all.
Besides, I wouldn't advice to make assumptions about the full name if you only know the diminutive. Some people prefer to use non standart diminutive of their name. (For example, I know a girl named Valentina, but she hates the standart short form Valya, and prefers to be called Lena (it's Valentina after all). So a lot of people assume that her full name is Yelena, but it isn't so) Also some people may have a name unusual for modern Russia and use a diminutive which is usually assosiated with another, more standart one. And there are even cases when the so called "diminutive" is an actual name.