The reason given in my book is that it i because it interferes with the secondary hemostasis..but i did not understand how that implies that bleeding from deeper organs is more common..
Skin is extraordinarily tough compared to much of our internal tissues; it has to be to protect us. To bleed, skin integrity must be breached, which usually involves sharp objects.
Think for a moment that most splenic or hepatic injuries (even complete ruptures) occur from blunt force trauma without breaking the overlying skin. For a more common example, if someone hits your thigh with a baseball bat, you will get a large bruise, maybe even a broken bone, but the skin will usually remain intact (if badly bruised) from blood vessel damage beneath and within the skin.
Many organs are delicate, and damage within them is not rare. Bleeding within/from some bodily organs is fairly common: runners bleed from their kidneys, and many, many people bleed from somewhere in their GI tract. High blood pressure may cause bleeding in the brain (a stroke) but it won't cause bleeding from the skin.
Just add to all the above the effect of anticoagulation (what heparin does.) I hope that explains things. If it doesn't, please leave a comment.