Yeah, my neighbor sent a letter to Hasbro a few years back with suggestions for their Star Wars toy line and they sent him a similar letter. It is a legal thing. Companies can't take unsolicited ideas for products from consumers. If they do, and they use that idea or something similar to it and make money from it, then the person who sent them the idea could potentially sue them. And if that person can successfully prove that they had the idea first, they might win.
It gets even trickier when dealing with a licensed product like Back to the Future. NBC Universal holds the copyright to Back To The Future and all derivative products, such as the film's score, the novelizations of the films, the animated series, and any and all video games produced in the franchise. Therefore, legally only people who have been given express written permission from NBC Universal can create anything Back To The Future related. Thus it's better all around if a company like Lego, which potentially could license the Back to the Future franchise, does not do so just because a fan writes and tells them they should.
Because if they do, that fan could try to sue them again, but this time there is no way the fan could win because the fan does not hold the copyright to Back to the Future, and therefore could potentially open himself up to a countersuit from NBC Universal for copyright violations. And no one, even NBC Universal, wants that.