Because the coaches wanted to keep the momentum or some such stupidity. There was never any doubt in anyone's mind who the better QB was, at least their shouldn't have been. And from the people who saw practice after he returned there was no doubt who was better, I read that from at least 3 different sources. They wanted Romo out because they thought Dak was 'good enough' and they'd groom him to be the next Russell Wilson - no cap hit and someone they could build around. They wanted Romo off the books because they believed they had a cheap option who would let them build around him. It never had to do with Dak being better than Romo, it had to do with Dak being cheaper than Romo. And Zeke hides a lot of warts, so the team was winning.
If Zeke had pulled a hamstring last season and missed 3 or 4 weeks we wouldn't be here now.
Edit: There is a reason they didn't let Romo compete for the job when he came back, and why they didn't offer to let him compete this year - because he would have won handily.