Here's why - it's not the songs, because the music in the game itself sounds frickin' awesome in every way - it's the mixing. The official OST release on digital music platforms sounds a little, well, off, and it seems the reason is due to how it's mixed.

The songs featured in the game were composed by Mick Gordon, who did a stellar job on the demonic soundtrack. It turns out though that Mick himself wasn't involved in the mixing, and isn't very happy with the results. 

According to @thatACDCguy on Twitter, the mix featured on the OST has less definition, meaning there is less dynamic range between the instruments. The upshot of that is that the music sounds flat, and instruments are harder to hear. It's a bit like a heavily compressed JPEG image - it's the same, it's just not quite as lovely.

Here's a comparison between the original BFG Division from Doom 2016's official soundtrack (left) vs. the BFG 2020 remix on Eternal's soundtrack from today (right).

Notice how the wavelengths in BFG 2020 form a nearly perfectly straight bar vs. the original with more definition pic.twitter.com/TCJRdOe1Yf

— Doominal Crossing: Eternal Horizons ? (@thatACDCguy) April 19, 2020

As mentioned, Mick Gordon himself didn't seem pleased about the whole situation, stating in response that "I didn't mix those and wouldn't have done that.  You'll be able to spot the small handful of tracks I mixed (Meathook, Command and Control, etc...)"

As for how or why this situation has come about, it's hard to say. There's seemingly no good technical reason for what has happened, but at least you can still enjoy the glorious soundtrack within Doom Eternal itself without compromise.