We often find ourselves working on the familiar things, things we have had repeated success with and things where we are least likely to fail. We often like to stay in our comfort zone, going with what we know.
That's the easy route. The projects may not always end successfully, but we go into them knowing more and start with a greater chance of success.
It's easy to start familiar projects with confidence, to win our clients over right out of the gate and start fresh with a new project team knowing we are going down a familiar path. The confidence we start these projects with shows to our customers, and it certainly makes it easier to get these projects off on the right foot because we know what we are doing - there really isn't any learning curve. But keep in mind there really isn't much career or professional growth involved, either.