people wanting to get back to the "main" topic they're interested in is a really common mistake i've noticed.
people are interested in X. X leads to Y which leads to Z. people are much less interested in Z than X, even though pursuing Z is the way to pursue X.
this is really broken. it gets in the way of making progress. it gets in the way of truth-seeking wherever it leads. it gets in the way of interdisciplinary learning. it means people want to learn only as long as the learning stays within certain boundaries.
here's one of my explanations of what's going on:
people want to work in particular fields rather than solve particular problems.
if your focus is purely on solving a problem (X), you'd be interested in whatever helps accomplish that goal.
but suppose instead your focus is on "i like woodworking. i want to work with wood". then you won't be interested in philosophy related to learning which could help with woodworking. cuz you want to do woodworking, not philosophy.
if your focus was on solving a really hard woodworking problem, then it'd lead you to philosophy and you'd be interested in philosophy because it helps with your problem.
i think a lot of people care more about what kind of activity they are doing – e.g. woodworking not philosophy – than they care about problem solving.
people have interests in topics (e.g. woodworking, dance, psychology, literature, architecture, programming, chemistry, politics) rather than having problem-directed interests.
another reason people lose interest is:
the more steps there are, and the more complicated the project gets, and the more tangents it follows ... then the more it's a big, longterm project. and they don't expect to successfully complete big, longterm projects. so what's the point?