Well, for one thing, planetary-based particle shields would defend against many such attacks. Shields similar to the one surrounding Scarif in Rogue One: A Star Wars Story were capable of deflecting anything from a shot from a laser cannon to a capital ship's turbolaser batteries, and as seen in the movie, torpedo attacks and even an X-wing starfighter slamming into the surface had next to zero effect on its integrity. Smaller meteoroids/meteorites would probably break apart and bounce off the shield, partially vaporized.
Now, that's not to say that mass-driver technology like modern real-world railguns and gauss cannons isn't used in Star Wars; in fact, it is and it's seen onscreen with the cannons mounted atop the Grand Army of the Republic's All-Terrain Tactical Enforcer walkers in Episodes II and III. In the Legends/EU continuity, other forms of mass-driver weaponry existed, too.
There was also an instance in Legends of an Imperial superweapon called the Galaxy Gun, which was an immense mass-driver weapon that launched a huge missile through hyperspace towards a target; the missile would exit hyperspace near enough to a planet for its own propulsion system to activate, sending the missile into the planet's surface and detonating a warhead that basically set off nuclear explosions throughout the world's atmosphere until the planet cracked and disintegrated in a massive explosion.
The simplest explanation for why your suggestion of meteorite weapons wasn't used, I'll frame in the form of a question: Why would you use it? If your goal is to cause an Extinction Level Event on a planet, why go through the hassle of hyperspace and dragging an asteroid when you have massive vessels hundreds of meters long with directed-energy and smaller ballistic weapons that can target with precision a base, a city, even a nation, and effectively turn it into a smoking crater? A single Imperial-class Stardestroyer had enough firepower to turn a continent's surface into an uninhabitable wasteland, a squadron of them could obliterate all life on a planet if so inclined.
However, there's no reason to suspect that the Empire wasn't researching that very concept. The Tarkin Initiative, the thinktank behind the Death Star's final designs and construction, is said to have had several other projects in varying stages of development in Rogue One, and so it's a logical possibility that a regime that created the Death Star (and in Legends created many more planet-killing superweapons that rivaled the canon Starkiller Base in terms of destructive power) wasn't looking into other new and exciting ways to destroy planets.