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，____________。
Another reason why …..(凑字数，但不可滥用) is that….(有利于写长句)
What is also worth noticing is that….
值得一提的是___________________。
There is another factor that deserves some words here.
还有另外一个很重要的原因。
A more essential factor why…. Is that
________ 的更为重要的原因是____________。
A further reason why,,,, is that…..
________的更深层次的原因是____________。
In addition..此外，再者
Moving on to wider themes,_______
展开来说，_________________________。
The second thing that must be taken into consideration is that…
第二个原因也很重要（值得考虑），亦即__________。
Likewise(同样), common sense tells us that…
