X-Men: Days of Future Past Trailer

I am just confused about one thing (well I am usually confused about many things, but that is neither here nor there):  if they were building Sentinels back in the 1970s, why is it that people in the 2000s still are in the dark about mutants?

Back in the first movie, they visit the president at the end and say that mutants are here to stay or something like that.  Despite this little thing, I am still pumped to see this film.