Compression: To optimize streaming and reduce bandwidth usage, SoundCloud may apply audio compression to the uploaded tracks. Compression can cause a loss of audio fidelity and detail, even if the original file was of high quality. Soundcloud is of the last platforms still using MP3 compression, for a large part of its catalog, even for new releases.
Uploading practices: SoundCloud allows artists to upload their music, and the quality of the uploaded tracks can vary depending on how the artists encode their files. If an artist uploads a lower quality file, even Go+ subscribers will hear the same diminished sound quality. It's quite common for artists themselves to upload degraded versions of their work on Soundcloud (shorter edits, reduced encoding quality)
Mastering: Sometimes, tracks uploaded to SoundCloud might not be mastered properly, leading to issues with sound quality. Mastering is the final stage of music production, where the audio is fine-tuned to ensure it sounds good on different playback systems.
Perception and comparison: If you are used to listening to music on other high-fidelity platforms or lossless formats, you might notice a difference when streaming on SoundCloud, even with the Go+ subscription. That's why it's difficult to migrate from Spotify, Apple Music, Deezer, Beatport, Tidal to Soundcloud Go+