Supposedly Zeus was going to be the original boss in PSASBR, but he was replaced with polygon man. So, in theory, they may have already had some assets done for Zeus, so he was easy to make. Also, it serves as a bit of cross marketing for Ascension.
There are many other characters they could've put in, but Zeus is the "Bowser" of GoW, so I dont mind too much. I think id be bummed out if they put in say, Hermes. That'd be really left field.
When i get Zeus, everybody will die.
We shall see who is the best ZEUS!!!
Also Issac and Zeus will come for free for whoever bought Ascension!! Link for whoever doubt http://www.ps3trophies.org/news/news-9213-PlayStat
Isaac is from Dead Space, which is a popular series and Zeus is the antagonist from the God of War series. Both make sense to be included.
But i'll admit, it would love it if they add Doolface.