After Halo Wars, the company that made it was shut down.
That would be Ensemble Studios.
Idiots decided to finish with the rather mediocre RTS game, on a console of all things. Didn't even bother to make a bad PC port, like Halo 2.
However, you can be sure, seeing as Microsoft and Bungie are going all out on this thing, that there will be a sequel. The money train never stops, and this is the most overloaded bandwagon out there.