The first answer is easy: some people still adhere to the Times and Seasons articles that Joseph Smith supposedly wrote or approved of. There are two basic problems with those articles. One, Joseph neither wrote nor approved of them. Two, the articles are patently false and ridiculous. No one I'm aware of believes Zarahemla is in Quirigua; the place doesn't even have a significant river. The buildings Stephens and Catherwood documented don't date to Book of Mormon time periods. And, of course, the articles misrepresent what the Book of Mormon says anyway. Attributing them to Joseph Smith is a huge error.
"Okay," people say, "we get that the articles are invalid. Why do some scholars still insist Mesoamerica is the setting for the Book of Mormon anyway?"
This answer is difficult for many people to believe, but it's a common academic response; i.e., the academics have been advocating the Mesoamerican theory for so long they don't care any more that it originated from these ridiculous articles. The Mesoamerican theory has acquired a life of its own. It doesn't matter that Mesoamerican proponents have to assume the Book of Mormon text is wrong. It doesn't matter that they have to assume Joseph Smith was speculating. And it doesn't matter that there is zero evidence of the Book of Mormon in Mesoamerica. They adhere to their theories because they are their theories!
Confirmation bias is a well-known problem and I could cite dozens of experts on it, but it's kind of fun to cite novelists, so here goes. Leo Tolstoy astutely summarized the problem over a century ago: