语法就是这么规定的 therefore就是个副词。这哪能有什么“为什么”
eg。He was down with the flu, and therefore couldn't come to the party.他患了流行性感冒,因此未能前来参加宴会。
and 是连词 连接两句句子(两个动词)
这句话也可以这么说He was down with the flu. Therefore he couldn't come to the party.副词时不能连接句子的。所以去掉了and 就要分成两句说。
参考资料:http://www.baidu.com/s?wd=therefore&ct=1048576