In Grade Three, we really up the ante when it comes to teaching children how to be effective writers. I've said this before, but this all lays the foundation for future grades when a large portion of their overall grade is dependent on written communication (e.g. exams, essays and reports).
So at this stage in their academic career, we teach children not to begin sentences with and, but or because. In later grades and with further experiences with higher level reading materials, students will learn when it is appropriate to begin sentences with these three words, but for now, we hold off on doing so.
Having this policy in place also puts that positive pressure on students to really consider a better way to begin a sentence. It's just so easy to start with "Because". Remove this as an option and we're forced to take things up a notch and consider new words or phrases.