Most services & streaming services compress audio files to make them download faster. An unfortunate side-effect is that compressed audio may not sound as good as the original tracks you uploaded to DistroKid.
As streaming services increasingly embrace uncompressed, “lossless” streaming, this will become less of a problem.
In the meanwhile, there's usually a way for listeners to make audio sound better. For example, in the Spotify app (desktop or mobile), paid users can go into "Preferences" and turn on "High Quality Streaming."