It's not a coincidence, it's just physics. Any rotating system like this will tend to sort objects by mass, with the heaviest at the middle. You can see this if you just take some sand and swirl it in a cup with some water. The largest grains will end up at the center. Why? They're the heaviest, and it takes the most energy to fling them around. If we simplify a galaxy to a two body system, you will imagine that a light star and a heavy black hole will orbit each other with the star making a much longer orbit than the black hole, which may be almost stationary. In a galaxy, you don't have a two body system, you have every body in the entire galaxy acting together, and relativistic effects and all that, but the same thing is happening. The distribution of masses in a galaxy broadly follows what's called the Einsato profile, which essentially says the densest objects will have the lowest radius. Of course, galaxies come in all shapes and sizes, and come in various states of development (a galaxy that has just collided with another may have a random distribution) but over time, a galaxy will organize itself with the densest objects at its center.