The above posts are certainly accurate, and let's also remember that the technology for stopping such a spill not only exists but it was installed on the Deepwater Horizon drilling rig - it simply wasn't maintained and once the leak/explosion had happened, it became much more difficult to stop.
Drilling at such depths is complicated and time consuming, not to mention expensive. So they are drilling relief well to take off some of the pressure of the leak so it can be capped, but that takes a long time to do, and the relief well has to tap directly into the original drilling shaft, which is only a few feet across. It's like hitting a needle in a haystack in a field of haystacks. The relief wells are the best option at this point, but they are by no means guaranteed to work.