There are several reasons why popular nostalgic music videos uploaded as early as 480p quality on YouTube are not converted to full HD or 4K despite having a huge fanbase. Firstly, the original source material for these videos may not be available in higher resolutions. Many music videos from the past were shot on standard definition video tape, which has a maximum resolution of 480p. In order to convert these videos to higher resolutions, the original film elements would need to be scanned in high definition, which can be a time-consuming and expensive process.
Another reason is that the cost and resources required to upscale old music videos may not be justified by the potential revenue generated from these videos. While these music videos may have a large fanbase, the revenue generated from ad views and merchandise sales may not be enough to cover the cost of remastering the videos in higher resolutions. Additionally, YouTube's default resolution for videos is 480p, which means that many viewers may not even notice the lower quality of the videos.
In conclusion, the reasons why popular nostalgic music videos uploaded as early as 480p quality on YouTube are not converted to full HD or 4K despite having a huge fanbase are due to the limitations of the original source material and the cost and resources required to upscale the videos. While fans may desire higher quality versions of these videos, the reality is that it may not be economically feasible for content owners to invest in the process of remastering old music videos in higher resolutions.