Like a drunken karaoke performance, Comic Con trailers have long been something studios love showing off in a particular, darkened venue but don't want you recording on your phone to show everyone later. This is odd, because a trailer for an X-Men movie is not at all like a whiskey-fueled rendition of "Rock and Roll All Nite"--save for both being evocative of heavy makeup and stupid black jumpsuits. A trailer is an advertisement for a product, and the studios selling it should be thrilled that there's such enthusiasm for seeing it.
20th Century Fox does not see it that way, though. They are sick of you assholes discreetly recording their commercials to show to other interested parties who were unable to attend the event. Thusly, according to The Wrap, they're pulling out of the big "Hall H" event at this year's Comic Con, as it's said that "the studio feels it cannot prevent the piracy of custom trailers."
In summary, now, instead of someone seeing footage at a promotional event, getting it on their phone, then tossing it on the internet to broaden the promotion, Fox will just post the video online themselves at a later date. Take that, pirates!