There is no known reason for that and no official comment have been made by the authors. Maybe some copyright issue or something similar? (the trailer featured Disturbed‘s Sound Of Silence cover)
We know that the trailer was higly appreciated by the entire fan base and by former Gears of War author Cliff Bleszinski as well, settling also a record on youtube in number of views (counting in millions) as confirmed by Rod Fergusson himself.
However at the moment you can still find it on some fans channel but we suggest to download a copy in HD format on your local hard drive for “personal use” if you like the idea to remain still able to watch it in the next future.