I’m guessing because it says it wants the past tense casual form at the top.
The hint seems to confuse matters though by seeming to suggest it will accept either the polite or the casual variant 
The problem is probably hard coded to only accept the casual form as the correct answer denoted by the [past tense casual] that is written on top. In these situations the [past tense casual] label will take precedence over the hint which may just be incorrectly written.
Sometimes bunpro will not accept the answer as correct, but also won’t mark it as incorrect resulting in saying something along the lines of “more casual!” and gives you another chance, but it doesn’t always happen.
In this case it’s up to you to be honest about it and choose whether or not you want to hit the “undo” button to re-enter the correct answer.