"Do you understand the meaning of this word?"
Because the word's meaning isn't the direct object of your understanding.
Sushi wo taberu -> you're directly acting on the sushi by eating it.
Kotoba no imi ga wakaru -> you aren't directly acting on the word's meaning by understanding it.
If you were saying you changed the word's meaning, then I believe you'd use wo.
I can understand that わかる does not take a direct object. But I don't understand why ガ -- a subject marker -- is used instead of は -- a topic marker. Does わかる work like 好き where the thing that is being liked is the subject, with a more literal translation of この本が好きです is that the book is likeable rather than someone likes the book? Does it work like この本が好きです, which more literally translates as the book is likeable, as opposed to someone liking the book? Would you then also say この本がわかります, showing that the book is understandable by someone, rather than someone understand the book?