Because you will not get the trade-in value that they have promised. I just sent a close to brand new 6 gold and it was rejected. After reading the Amazon forums on this, the trade-in company have a knack for rejecting a phone from whatever condition you pick.
One guy sent in as New, and they rejected it saying it’s just Good. He then shipped it back marked as good, and they rejected it saying it’s only acceptable.
Never ever select the option where they get to keep it at the lower price; instead pick return to sender. The other thing to keep in mind is that this service is provided by a 3rd party, which is not owned or supported by Amazon.