Another possibility: life came to Earth already well formed (panspermia), and that single ancestral introduction rapidly filled the terrestrial biosphere so no further introductions could take hold.
Assuming that abiogenesis wouldn't happen very frequently (given the right set of conditions, a mean time to happen on the order of thousands to billions of years), a single life-forming event would have the same effect, no? After a foundation is set, the self-replicator starts exploiting existing energy gradients, making any other abiogenesis-event much less likely.
Panspermia is such a plausible theory. We know that life spreads like crazy if you let it -- try to keep something sterile and life always finds a way in, even into our sealed jars and cans, volcanoes, the bottom of the ocean, etc. I wouldn't be surprised if one day we find that Mars is teeming with bacteria that hitched a ride on one of our rovers and adapted.
Almost certainly Mars would have been seeded billions of years ago, w. bacteria hitching rides on ejected debris from asteroid impacts, when conditions were much more hospitable there. And of course it could have gone in the other direction also. Even autopanspermia -- where ejecta from a planet-sterilizing megaimpact later comes back to the cooled off world -- is a possibility.