I don't think it's quite that simple, though I suspect there is a grain of truth to it, that apolitical or less emotional music is manipulated out of popular and financial success. Mostly, though, I think it's just the nature of reaching wide audiences. The "blander" (here meaning simply not particularly heavy on any subject matter) something is, the fewer people will be put off by what it has to say. If all there is to a song is just enjoying the piece of music for entertainment, there are simply more people who would appreciate that over, say, black metal, that is designed to evoke certain specific strong emotions

I also suspect that your premise is not so much flawed, but a disingenuous oversimplification, and that popular music probably involves heavier and "angrier" themes then you are giving it credit for. Or maybe you're right entirely. I couldn't name a single Taylor Swift song. I'm a metalhead, and whenever I go outside that comfort zone, it's never to pop music. I look for artistry in music, and the top 100 don't guarantee artistry, only sales, which if you wanna get into, is a whole lot of conspiring, just not political

Sales as a measure of success is entirely flawed in a capitalist society well after the invention of the field of psychology. When you know how to manipulate people, you can manipulate populations into buying shitty music. If you have the money, you can pay for advertising, which will make your artist seem more popular than they are, and then give you even more money to pump into yet more advertisement. It is certainly political, but not in a "we can't have people having revolutionary thoughts" way, just in a "capitalism must perpetuate itself" way