Main reason is nearly no one needs to measure things in megameters. Megameters would be a unit to measure the diameter of planets in, maybe the orbital altitudes of some moons. Our moon for example is ~384Mm away. Distances between planets, distances between stars, and distance between galaxies are many, many orders of magnitude farther than that.

As most of us rarely travel more than 1,000 kilometers very often, it’s the biggest unit most people are familiar with on an intuitive level.

I’m still convinced people don’t actually use the metric system’s power of ten design. Like no one uses centigrams or kiloliters either. They’ve picked out units that are pretty close to the ones in the Imperial/Customary system, kilograms are used instead of pounds, grams are used instead of ounces, kilometers are used instead of miles, meters are used instead of yards, centimeters are used instead of inches, millimeters are used instead of sixteenths of an inch and so on. Want to confuse a European? Draw up some blueprints in hectometers.