First we had to have the audio processor. Good EAX was available on top of the line cards, and they were not always cheap. Lower end chips got less features.
Then we had to have the speaker setup to have the greatest sound, or needed to get a real 5.1 headphones, which were bulky and never provided the same fidelity.
Then Microsoft changed the Windows driver model, cutting the driver's direct access to the card. All of the timing sensitive effects were gone in an instant. I remember installing the new drivers and getting literally nothing. Sound Blaster was the only card with an hardware mixer, and Microsoft didn't feel like enabling them. Mixing at the DirectX layer killed the cards.
Soundblaster's very closed stance didn't help them either. None of the cards after Audigy2 worked with Linux when I had my desktop system.
After my Audigy2ZS, I moved to Asus Xonar D2X. Its positional audio capabilities were nice, but I mostly bought it for its Linux support and sound quality, and that was top notch in that regards.
Then sound cards became commodity. Everybody stopped making good cards. Musicians moved to audio interfaces, audiophiles moved to DACs.
Just looked to the SoundBlaster website. Internal cards are very limited. One DAC, one DTS enabled 7.1 sound card for PC cinema systems, three game oriented lower end cards, nothing else.
It wasn't really the cool reverb effects or wave tables, though those were a nice bonus. It was just "I can tell my computer to make sound and it actually makes sound without days of troubleshooting."
Granted, similar things could be said about 3dfx. It's was a 3D card with drivers that actually worked.
And then there's the obvious "sound blasters and voodoos go in my computer, jalapeno goes in someone else's computer" thing.
In fairness on board (depending on the board but on the whole) is pretty good.