I know the thread is old, and yes, Kopernicus is the problem of this "bug".
But in case anyone finds it and wants to fix their problem with Kopernicus disabling asteroid spawning by default, go to:
Steam\steamapps\common\Kerbal Space Program\GameData\Kopernicus\Config\Kopernicus_Config.cfg
UseKopernicusAsteroidSystem = true
UseKopernicusAsteroidSystem = stock
After the change i got 2 asteroids within 30 seconds of restarting my game, but keep in mind, the setting is only supposed to be used with stock-like solar systems (like OPM)