Oftentimes, when you have a weapon holstered on your back, it will clip into your back pretty badly. Sometimes it gets very bad (For example, the SG5-K will clip so badly the magazine will show through your chest. Some guns are almost completely swallowed by your back.). They would look great on your back if this wasn't such an issue.

This can also be a problem gameplay-wise. Ghillie wrap is made basically unusable in first-person only servers. When you have it equipped to a weapon on your back, it blocks at least 15% of either the left or the right side of your screen (depending on what shoulder your gun is equipped to.)