Because

we were wondering about it last night as we were watching the show on Voyager.

How big a change in the Earth's orbit would be required to destroy all life?

"However, to destroy all life on Earth the orbit would have to change by at least hundreds of thousands of miles"

Sounds like a lot right?

But when you consider that the earth is an average of 93 million miles from the sun?

900,000 miles would equal .0096 of that distance or .96%

Less than one percent of the distance further or closer would destroy all life.

Another way of looking at it?

The Distance between the Earth and Venus "on average, it is 25 million miles".

The distance between the Earth and Mars is "on average 139 million miles".

Less than 1% room for error.

Seems to me like we're at the right place :-).