Getting the question right

It’s lovely to hear students being sociable, especially with those who are new to the class. I’m especially happy when they try using some of the language discussed earlier during the course. It’s true the language is not always used correctly but full marks for trying. I’ll show you what I mean:

Can you tell me why did you come to Malta?

The student wanted to be polite and therefore used an indirect style of asking. While the meaning is clear, grammatically it isn’t correct. What is wrong? Why was the error made?