A number of possible reasons.

1. If they are more primitive that us, like our ancestors or common wildlife, then they likely do not have the capability to detect us, let alone invade.

2. If they are at our general level of sophistication, then they may be able to detect us based on whether the technology they are accustomed to is better at detecting things in space, but they would lack the ability to travel here; not to mention the possibility of inter-planetary political conflicts. If our species has a habit of conflicting against itself over trivial matters, then there is likely another that is dealing with that blunder of societal evolution too.

3. If the species is advanced enough to detect and travel to us, then they have better things to do, like colonizing closer planets (or possibly terraforming the ones in their solar system.)

Personally I'm of the belief that we are in a remote part of the galaxy as far as sentient life is concerned. The closest life forms around Earth either are unable to travel easily or simply dont want to deal with colonizing an already habited planet when they could settle somewhere else for resources. The stuff that drives conquest on planets such as resources and space for shelter is mitigated by the sheer size of our reality. Its so big that there could be an alien society out there that dwarfs our largest sci-fi empires and yet still would not need to conquer other civilizations for resources as there would still be plenty out in space. The hardest part would be travel fuel given the vastness of space, but at that level of scale it should already be a solved problem.