This might sound silly, but could it be because the DX is the lower grade of disc, and the Champ and Star are more serious discs. Most people who throw Champ and Star don't care about the stamp.
I think this is probably the case.
I learned this the hard way, buying a new Wraith for my daughter - I thought, "hey, it's her birthday, get her a champ," so I did. She was really disappointed that the "Wraith" character was not on the disc.
New/Young players buy discs based on marketing - they buy what seems cool because they don't know any better. There must be a market out there directed at people who are only going to play one round of disc golf. Those people buy discs, play a round, and either lose their discs or leave them to sit in the basement forever.
To most of us (exclude the disc dying community), it doesn't matter what the disc looks like - we're concerned with the flight characteristics. Therefore, we buy what we need the disc to do - regardless of what it looks like. If I gave you a guaranteed upshot sinker from 200', but stipulated that you had to throw it with my purple disc with pink poke-a-dots, I'm guessing that you would take that hit.
To make amends to the disc dying community: they are taking discs that they care about in terms of flight, and making them look like they want them to look.
I think the logos and images are for newbies: sell discs to those only marginally invested in disc golf.
Incidentally, I still buy DX discs, especially with putters for the "stickiness" of the plastic when it hits the chains.