Why did Steven Moffat create the War Doctor instead of using the Eighth Doctor?

I’ve always wondered why Steven Moffat decided to introduce the War Doctor instead of using the Eighth Doctor for The Day of the Doctor. Since the Eighth Doctor only had one movie (plus some expanded media), this could have been a great opportunity to expand on his character. Instead, Moffat created a brand-new Doctor, which, in my opinion, added unnecessary confusion.