They explained a lot of the stuff in the advertising campaign and on their sites. Also, they (rightfully) choose not to explain a lot of stuff for multiple reasons. You can't realistically explain something so fictional as a giant monster that comes trashing NYC.
Secondly, not explaining things leaves the viewer in the unknown. The unknown is often more frightful than an explanation.
Also, the whole idea of the movie was that you were watching a handycam. You only know what's recorded on the cam.
As for the explanation, IIRC, the monster came from the deep in the sea where humans haven't ventured yet. Some fictional company went there to collect an ingredient for their product (the site and the product even had websites and offers) and angered the monster, who was actually still a baby (hence it going all apeshit). In a part of the recording you can see the company's satellite crashing into the sea a few days prior to d-day. That's when the company's drilling island got wrecked by the baby monster. Only one tanker manages to get away. That's the tanker you see get destroyed first when it enters the New York harbor. The makers of the movie actually mapped out the entire trail the monster walked and it corresponds to the movie (note the Statue of Liberty etc.).
There was more, but I forgot some things.
Anyway, to each his own of course.