It’s probably more because philosophy in general isn’t something that lots of people have an interest in, economics are often boring, and Das Kapital combines both into one incredibly dense, challenging to read book.
Personally, I think that Marx and Engels do a great job of describing the problem, but I don’t think that their solution takes human nature into effect. Economics assumes that people are fully rational actors that will do the smartest thing all the time, but that’s clearly not the case; there are other forces shaping human behavior. Marx and Engels don’t seem to account for that.