They had their best suits on because of the celebration. 原因状语
They had their best suits on for the celebration. 目的状语
They had their best suits on (since / as) the celebration. 错误
造句时要用你学过的、见过的结构,不要自己创造。这就是为什么我们要多阅读,熟悉常用的表达。
since作为介词,不能表示原因。
as当“作为”解时,有时可以翻译为原因,但并非真正的原因状语,而是表示身份等。