The proverbial buck has to stop somewhere, and in the case of Melania Trump and the Plagiarized Speech, it's with Trump speechwriter Meredith McIver. According to the Washington Times, McIver offered to resign from the campaign, but Donald Trump refused to accept her resignation, describing the plagiarism as an "innocent mistake."
McIver allegedly said,
"I apologize for the confusion and hysteria my mistake has caused. . . I did not check Mrs. Obama’s speeches. This was my mistake and I feel terrible for the chaos I have caused Melania and the Trumps, as well as to Mrs. Obama. No harm was meant."
According to McIver, Melania Trump was reading passages of Michelle Obama's 2008 speech at the Democratic National Convention to McIver over the phone during the writing process. McIver said she wrote fragments of the speech down, and later included them in Melania Trump's speech.
This doesn't really explain why Michelle Obama's words were used in Melania Trump's speech, unless McIver is saying she somehow forgot that the words she wrote down weren't actually her own. But whatever, this is probably as close to an explanation as anyone will get.