The reason is that the faster the line is pulled from the reel and the more line is in the water, the more friction there is one the line and the more pressure on the line. Also, you can get busted off on the strike due to inertia of the spool etc.
The recommended drag pressure is 33% line strength. You can try increasing this, but the higher you go the more you will get busted off.
My thoughts anyway
Jeremy