We know enough about this asteroid to know it is a potential threat but we still have a few more years to watch this thing and get a better handle on what it is going to do.

It's like changing the angle of a bank shot in pool. A little change early on has a large effect down the line.

You just want to hang there and let the spacecraft gradually adjust the asteroid's course.

By using gravity as your tow line, you can sidle up to an asteroid. Maintain it for a year and that should give it enough nudge to miss the earth 20 years later.

Impacts and explosions are difficult to predict and control. When you're trying to save the Earth, you want them to be both controllable and predictable.