Because it depends!
Joke aside, I find that the most difficult challenge developers struggle with is that most things that really matter in terms of impact on the success of a software project depend on guidelines, patterns, best practices and whatever other name you may call it.
Every time I hear someone defend some piece of code with a pattern or a guideline I keep hoping to hear the real justification behind it, like what does the project gain from adopting this guideline or following this pattern, but people generally stop at the pattern/guideline reference.
C’mon folks, that’s not enough, we call them patterns, guidelines and best practices for a reason… they are applicable in a given context and will most likely help you, but they cannot be applied blindly. Otherwise, they would be called rules and software development would be a breeze.
For example, just because someone says to prefer interfaces over abstract classes you should not hardcode in your subconscience that an interface will solve all your problems. In the same tone you can always solve a problem by adding another layer of abstraction, except when your problem becomes to many layers of abstraction.
Another one of my favorites is the quest on eliminating any possible duplication. Don’t get me wrong, I totally believe in DRY but you need to have some common sense because if two things are the same now but may change due to completely different reasons you should carefully evaluate if you gain anything from trying to avoid the so-called duplication.
You should do stuff that helps you tame the complexity of the software you’re developing even if it contradicts one or more guidelines or best practices. If you want to know more on this subject, you should read Code Complete by Steve McConnel, it’s a pretty big book, but totally worth it.
Exceptional Code 
You forgot another point. The software development is a pain because of the specification! 🙂
“Walking on water and developing software from a specification are easy if both are frozen.”