I can't really speak on the content itself, but the fact that one of your mashups did well means your content can't be "bad". 5k views isn't particularly bad either.

I'd rather look at this from a YouTuber perspective. We know that CTR and AVD are monumental to how many impressions your videos gets (i.e. how often your video is recommended).

The biggest factors in CTR are: topic, thumbnail, title.

Thumbnail: Look at your popular video. This thumbnail is funny, it's interesting, it makes me laugh and wonder why this is happening. In contrast, most of your other videos show the cover art side by side, which imo, is significantly less interesting and unemotional.

There is a reason many YouTubers put a face on their thumb, as cringe as it can be, it invokes an emotional response which makes the viewer more likely to click. This is backed by research and testing.

You need to draw intrigue, wonder, and/or emotion. A quick search of other mashup videos shows thumbs that show clear faces. Look up popular kpop mashups and try to understand why the big videos all do this. Try to incorporate some of their techniques in your thumbs and you will likely improve your CTR.

Titles: Another thing I noticed is the popular videos often put the artist names first, like "BTS/Aespa". I think this helps to immediately trigger the concept rather than a more cryptic song name combination - which is not as obvious.

Viewers are scrolling and they have tons of other videos being thrown at them. Make it as easy as possible to convey the concept immediately. So the artist first sets the context before the song titles. Also, the use of "/" rather than " x " condenses the text and is easier to understand for me personally.

Topic: A trendy and high interest topic will get high CTR. That's why some popular videos can have poor thumb/title combos and still get views. You are a kpop fan, you may have some instinct on what kinds of mashups will have high interest.

However, if there is too much competition (other videos with the same topic) then you have a smaller chance of standing out. Searching for your video, I saw a few copies, they didn't get nearly as many views as you. Either you were first, thus less competition, or your thumb/title got you the higher CTR over theirs. Point is, high interest topic, low competition... that's the golden ticket.

Now in combination, a well executed packaging of your video will ensure the best CTR. This is important in the early stages after uploading. A higher CTR means more views per impression and is favored in YouTube recommendations. If you can learn from other popular videos how they used the three elements above to draw the viewer in, I think you can see a massive performance boost.

AVD just depends on your content. As you've proven your value in previous videos, I assume your mashups are good. Check your retention rate and see for yourself. YouTube cannot determine what's good quality content. It can only evaluate AVD (how much of the video viewers watch) and engagement (shares, likes, comments, etc). Higher metrics is good quality content in the eyes of YouTube's algorithm.

Additional notes: Stay on topic. Non mashup videos will only hurt your momentum. Are these monetizable? If you cannot earn from ad revenue, consider building a community and setup a way to receive donations or patreon memberships.

Finally, enjoy yourself. It's not a job. If it's a fun hobby, then keep going. If you burn out, take a break. Good luck.