Hello Tonyoung
I'm sorry but I'm Japanese. In my language we say "The reason is that ..." or "It is by the reason/cause that ....".
By the way, I found a similar question and the answer to it in
Webster's grammar QA site.
[Question]
When my mother was in high school she had a teacher that would would give her an F on a paper if she used "is because," for instance, "It is because of the sun that there is light on the earth." I have no idea why it is wrong or even if it is wrong, but every time I say "is because" in my mother's presence she corrects me.
I have tried to do some research on the Web to find the answer but have had no luck. The only thing that I could find was something that said "the reason is because" is wrong because it is redundant and should be changed to "the reason is that." This still doesn't answer my question because I am concerned about why my mother thinks the word "is" followed by the word "because" is incorrect in any circumstance. Your help is appreciated.
[Answer]
This is the first I've heard of this objection. Burchfield speaks directly to this issue: "It is also in order to use because after an introductory , , , ." And then he gives several examples from "reputable sources." The fact that Burchfield brings this up, however, suggests that there must be some writers who object to the construction. Perhaps they're objecting to a clause that explains why coming after a linking verb? Or perhaps, as you suggest, it's a carry-over from a quite reasonable objection to the redundant "the reason is because." In any case, Burchfield says that "That's because . . . " or "it's because . . . " is acceptable in both speech and writing.
Authority: The New Fowler's Modern English Usage edited by R.W. Burchfield. Clarendon Press: Oxford, England. 1996. Used with the permission of Oxford University Press. (under "because")
paco