There was no advanced force in the book. And they never showed any battle, They made it look like the UNSC didn't have any space defences.
Halo reach took place on the planet Reach. While the Space battles were taking place, Noble team was on the ground. They don't mention that chief is even on the Pillar of Autumn, and the ship was never docked on Reach when the invasion started.
The ship docked as the planet was being overwhelmed. Chief was already aboard. Red team should have been mentioned, considering they were the only reason Reach groundside didn't fall instantly. (Spartan II's, no III's)
Where would they even fit in? And it had been based off the book, yes it would have been a better campaign. They already established good characters in the book, they didn't need to make all these new characters that aren't interesting.
Spartan IIs are flat characters, that is part of their character. Point is, what would the game be about? All the Spartans did was defend Mac generators. And Reach falling in hours makes more sense then it falling in 2 days. The fleet that the Covenant had was significantly bigger than any seen before, and the way the UNSC fought them was in a back and forth shootout using repair platforms to take 3 hits. Considering how quickly the UNSC were losing ships, that battle would last a few hours, also at the same time Reach's under siege from thousands of Covenant dropships.
Maybe it was a day and early hours of the morning?
Btw the Unsc still has a massive defence force on Reach, it would have taken some time for the planet to be almost completely overwhelmed.