The most likely reason is that basalt is an extrusive rock, while granite is an intrusive rock. What this means is that basalt forms at the surface of the earth, where temperatures are cooler. Because of the cooler external temperature, basalt cools relatively quickly, and there isn't as much time for mineral crystals within the rock to align to form larger crystals while the rock is cooling. Granite, on the other hand, forms below the surface of the earth, where temperatures are significantly higher. Therefore, it doesn't cool as quickly, and the minerals within granite have more time to form larger crystals and the rock cools.
Temperature. Water is an example. When water is at room temp. its liquid. When water is at boiling temp. It is a gas. And when water is at freezing temp. Its a solid.
