This is the 86th most frequent Chinese word.
因为
"Because" or "due to."
In this sentence, "因为" introduces the reason or cause for the action taken, which is canceling the picnic.
因为天气不好，我们取消了野餐。
Because of the bad weather, we canceled the picnic.
Here, "因为" is used to explain the cause for the preceding statement, namely her diligent studying.
她学习非常努力，因为她想考上好大学。
She studies very hard because she wants to get into a good university.
In this example, "因为" is used to provide the reason for the situation described, which is being late.