英语表原因的长句有多种表达方式，以下是一些常见的句型和示例：

The reason why… is that…

示例：The reason why he didn't come is that he was sick.（他没来的原因是他生病了。）

Many factors weigh heavily for…

示例：Many factors weigh heavily for the decision to be made.（对于这个决定来说，很多因素影响很大。）

It’s only the surface explanation for…

示例：It’s only the surface explanation for the complexity of the issue.（这只是对问题复杂性的表面解释。）

Another contributing factor/cause is

示例：Another contributing factor is the lack of awareness among the public.（另一个影响因素是公众缺乏意识。）

The reason behind this decision is due to the fact that…

示例：The reason behind this decision is due to the fact that the company has been facing significant financial challenges over the past few years.（这个决定的原因是因为该公司在过去几年里一直面临严重的财务挑战。）

There are three reasons for this.

示例：There are three reasons for this: first, the new policy is not well-received； second, the implementation process has been problematic； third, the expected outcomes are not as good as anticipated.（有三个原因：第一，新政策不受欢迎；第二，实施过程有问题；第三，预期结果不如预期。）

The reason for this is obvious.

示例：The reason for this is obvious: the evidence points to his guilt.（原因很明显：证据指向他的罪行。）

The reason for this is not far to seek.

示例：The reason for this is not far to seek: it lies in the fact that he has been neglecting his duties.（原因不难找：事实在于他一直在忽视自己的职责。）

A number of factors are accountable for this situation.

示例：A number of factors are accountable for this situation, including economic downturn and political instability.（许多因素导致了这种情况，包括经济衰退和政治不稳定。）

The phenomenon mainly stems from the fact that…

示例：The phenomenon mainly stems from the fact that climate change is accelerating the melting of polar ice caps.（这种现象主要源于气候变化加速了极地冰盖的融化。）

这些句型和示例可以帮助你在不同的语境下表达复杂的原因。根据具体情况选择合适的句型可以使你的表达更加清晰和准确。