If it ain't broke don't fix it. If it is broken, just throw it in the trash can.
Speaking to Edge, Firaxis Lead Designer Jake Solomon explained that the first attempt at making XCOM: Enemy Unknown in 2003 ended up a disaster. He said:
It was awful. It’s a good thing it never went ahead, because I was way too young, I had very little experience, and I just wasn’t in the right place to make that game. It really took a long time until it made sense for the team and for the company.
Firaxis ditched the project and immediately began working on Sid Meier's Civilization IV which was far from a disaster. It wasn't until nine years later that XCOM: Enemy Unknown would come to fruition, and similar to Civilization IV it was successful commercially and critically.
Not many games get the luxury of being moved to the side until a development team is ready to tackle its challenges. XCOM: Enemy Unknown is a rare example, and it's unsurprising that its unique style was difficult to execute upon. Think of it, there's little to reference for the development team since the genre is spread so thin, and there are few lessons to learn from other developers. It's miraculous that it came out as well as it did.