The designers haven't said anything about it.
That's the direct answer to your question. As to why, since they haven't said anything, the best I can do is extrapolate. To that end:
Convenience
That's really it. Imagine for a second that this rule wasn't in place. Firebolt is a cantrip which ignites flammable objects. So I make a Sorcerer who specializes in twinning firebolts and my entire purpose in life is incinerating everything an enemy has on him.
Ogre coming at me with a club? Firebolt! Good bye club.