The principal reason for my change of mind is this.

我改变主意的最重要原因就是这个。

《柯林斯英汉双解大词典》

The principal reason for this omission is lack of time.

跳过它的主要原因是时间不足。

《牛津词典》

It is a principal reason at the least.

至少这是一个主要的原因。

youdao

One, it helps them relax and relieve stress, so this is perhaps the principal reason.

第一,它帮助人们放松 舒缓压力,这可能是主要原因

耶鲁公开课 - 聆听音乐课程节选

So, my perfect pre-med/bio plan was getting a little shaken right from the start, and the reason that it was getting taken was because I would learn this new principal in chemistry and because I was taking bio with the same time, I could see the connections.

所以我的完美的医学预科,和生物学的计划，从一开始就小小的动摇了,原因就是我一边学习这些化学原理,一边学生物,我可以看到两者之间的关联。

麻省理工公开课 - 化学原理课程节选

The political correctness mentality was the principal reason why Fort Hood's alleged murderer, Maj.

CENTERFORSECURITYPOLICY: Center For Security Policy

Asked about 11 September, he said the principal reason for it appeared to be US foreign policy.

BBC: Obituary: Chechen rebel Khattab

That means domestic policy can't be the principal reason for the current spread between high earners and others.

WSJ: Allan Meltzer: A Look at the Global One Percent