The first and most important reason is that.....

第一个而且最重要的原因是______________。

One of the reasons that are given for....is that....

______________的原因之一是__________________。

Perhaps this is because of the simple fact that.....

也许，原因很简单，亦即_____________________。

One of the premier causes is that(注意用词)

最重要的原因之一便是______________。

At the risk of sounding too simplistic, it seems to me that the main propositions can be summarized in one saying.

这个原因可能较简单，不过，我认为可以用一句谚语来概括，________。

We may look into every possible reason, however, foremost reason for .... Is ....

其实有很多原因，不过，造成________的最重要的原因是______________。

It might also be noted that....

需要注意的是，____________。