The game is CPU-bound on some of its rendering, so reducing the load on the CPU by using uncompressed audio allowed them to maintain graphical fidelity with reduced minimum requirements in terms of CPU.

If it's stupid and it works, it's not stupid.

There are plenty of high-speed decompressors that could possibly be adapted to PCM data, though, I'm sure RAD would sell you one.

I think there's also a healthy amount of "People think more GBs equals more content."