There are many reasons as to why some stories you write don't reach the end.
1). You don't actually want what you write. Maybe you just got carried away by the trend, or forced to work on it coz you see its popularity. Or you're just in for the fame and money. When those didn't work, it is only natural that you lose interest in it.
Solution? You may need to reorient your goals why you write.
2). You don't have any direction of what to do with those stories. This is similar to authors here asking what will be the next for a certain part of their novel from random people at the forum. Sure, receiving answers might revive your interest for a time, but it's technically not your work, and you lose interest.
Solution? Plan your stories. Have a vision of its start and ending. There will come a time that you will lose interest, but what separates the consistent author from others is that, they return to work on it, even if it takes a while.
3). In an attempt to keep churning out chapters, you keep writing even though the signs of exhaustion or burnout is there. Then, once burn out comes, you just stop, unable to continue. Few years or months past, you lose your original vision of the work and the interest drops.
Solution? Knowing your writing habits and planning with that put into consideration will help you. I'm a lazy writer, see? But because I know my writing strengths, weaknesses and routines, I developed a workaround for it. As a result, I just finished my 20-book series started in May 2019.
Now, about your novel that you have an on-and-off relationship, it might be that it's one you really love to write, or it can also be the story structure itself. An episodic work tends to be resilient to long pauses/breaks than a long, consistent one.