tiny diamonds in meteorite "signature of a long-lost planet": why come to Earth in 2008? ref this paper in Nature re the Almahata Sitta 2008 meteorite shower.

I get that they're saying the diamonds could only have been formed within something the size of planet, which subsequently broke apart in the "early days" of the Solar System.

I get that Saturn and Jupiter are so massive relative to other Solar System bodies they've marshalled fragments into the Asteroid belt, Kuiper belt, outer disk. And that they're still disturbing those regions enough to occasionally pull things into a cometary orbit/become meteors.

This putative planet must have scattered everywhere when it first broke apart/lead to comets and meteors all over the place. But why/how did this fragment wait until 2008 to come to Earth? Where's it been for billions of years? Why didn't it either crash into something a long time ago; or form itself into a stable orbit?

Possibly related, as in: why haven't meteorites originating from early-formed planets all long gone extinct?
Many comets are such lucky ones...
That's why we see them with their icy tails evaporating: They have only recently been scattered into the inner solar system.
Just like the well-studied comet 67P, you could ask "where has 67P been for all those billion years?". For at least this comet, we know the answer: It has been orbiting in the outer solar system, until a close encounter in 1840 and a few subsequent later ones with Jupiter brought its' perihelion closer to the sun, where it started its' cometary activity.

I would place my bet on the origin of those comets being in the outer solar system, because:  


The nature article you cite mentions 20 GPa pressure, for which one needs at least a Mercury or Mars type body to generate. This is not necessarily a problem, because Mars-type bodies form quick and early (we know this from theory as well as data), within the first 3-5 Million years of the solar system, when gas is still there. This is about 10 times faster than an Earth-type body, which has 10 times the mass.
It is also thought that Mars-type bodies should form the basic building blocks for larger planets - thus there should be many more of them in the early solar system.
However even if there are many, now there are essentially none in the inner solar system - the asteroid belt contains roughly 0.1% of a Mars mass. The total mass of the Kuiper belt is estimated to be far larger, by factors of 10-100.
If a giant planet instability occurred, it requires a even much more massive Kuipter belt as trigger, after the gas disappears in the early solar system.


So, Summarizing: They're probably out there in the outer solar system, merrily orbiting along.