Oh, that is known.
Out of all the rocky inner planets, only Earth still has a magnetosphere. The ones in the other three inner planets died billions of years ago.
However, in this aspect Earth was lucky. Or should I say "Earth Mark II", as we are really standing on the second planet that has occupied this location.
"Earth Mark I" was the original planet in this orbit around the sun, but it had a trojan. A roughly Mars sized planet that orbited at one of our Lagrange Points, and would have been there for hundreds of millions of years. However, those are not stable in the long term and eventually gravitational influences caused it to crash into Earth. Hence, Earth Mark I was destroyed, and Earth Mark II was formed. A now larger planet with a large moon orbiting it in comparison to the moons of other inner planets.
But here is what has likely saved the Earth. Much of the crust of Earth and Thea was blown off, some returning to the planet to form our crust. While much remained in space and formed the moon. But the inner core of Thea sank to combine with the core of Earth Mark I to give us our current core. Ultimately, the core that is "super sized" in comparison to the other cores of the other rocky planets. This is only being realized fairly recently, as it is the key to something that had puzzled volcanologists and geologists for years.
For decades they had known of "Large low-shear-velocity provinces" (LLSVP), which are large molten extensions off of our main core, with no logical reason for being. Some of these were already known, like a large "magnetic dead zone" between Africa and South America. And by the 1980s scientists were already puzzling how big they were, and where they came from. But it is only in the last 3 years that they started to realize what they are seeing is the remains of the core of Thea. Broken up and resting on top of our own core. And we now know it is different, because this "foreign core" is even higher in iron content than our own core is.
Now, take an Earth sized planet and throw it at Mars about 4.5 bya, and it too would still have an active core and magnetosphere. And likely a thick atmosphere, surface water, and possibly (actually likely) to have some kind of life. But without that, at roughly the time that Thea and Earth collided the core of Mars was already cooling and the magnetosphere dying.
We have known for a long time that our planet was unusual, in that our core was far larger and more active than that of Mercury, Venus, or Mars. But that was the final piece of the puzzle that fully explained what was so unusual about it. But there are other things known about the two planets, that are even more puzzling. Like Thea was unusually high in iron, which is why the remnants on our own core are so strange magnetically even billions of years later. And our own core is unusually high in uranium, making it hotter than the core of any other rocky planet known.
We now know all these things, but as to why, we will have to ultimately discover where the Sun and our Solar System was "born". Because yet another mystery yet to be solved, is exactly what nebula our star was born in. After roughly 5 billion years we have long ago left it behind, so we do not know the composition of the elements that were used to make up our system. We only know about our own planet, and what we can observe from looking at the surface of the other planets (sometimes).
And many are now starting to connect these LLSVP with plate tectonics. This if proven through analysis can help explain why unlike all of the other planets we still have an active crust, and why it continues to move and shift to this day. The crust on all the other rocky planets solidified over 4 bya, and have remained relatively unchanged since then. Even mars was solidified by 4.5 bya, the volcanoes did puncture the crust, but the crust was already hard even by then so is actually older than the crust of the Earth. That is why geologists get so excited when they learn new things about the rocks of Mars. Or the Moon, as they at most only date to around 4.2 bya. But the rocks of Mars are much older, possibly 4.5 to 5 billion years. Between Thea and plate tectonics no rocks of that age still remain on our own planet.