Nearest neighbor doesn't necessarily mean a near neighbor. :p (Technically Venus is closer, but in the other direction.)
Compared to Earth, Mars is ~1.52x as far from the Sun, which is a pretty hefty jump!
As you travel the distance from the sun by a factor of R, the same sunlight energy is distributed across a broader "shell" that grows in area by R^2.
1.00^2 / 1.52^2 =~ 43.3%
I can't say it very well so I'll leave it to a great:
Space is big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space. - Douglas Adams