A lot of people haven't even mentioned the flood. The flood is a basically unstoppable parasitic life form that can kill of an entire galaxy with little effort. If they became sided with the rest of the halo universe, the flood would be the primary factor in the fight on halos side. Star Wars simply does not have the resources to fight the flood in addition to the rest of the halo forces right behind them. The flood would just infect entire worlds and take over everything. Star Forge? Infected. Death Star? Infected. Jedi? Sith? Clones and mandalorians? Infected. Don't even get me started with an infected krayt Dragon