We can assume it's some form of weird contractual obligation and/or TPCi being overly protective of their brand, but it's genuinely concerning how their licensed Pokemon product lags behind other TCGs they have licenses with.
I'm all for criticizing how TPCi handles its own franchise, but this probably isn't it. We know TPCi approves original artwork for the Japanese merchandise, and we also see some pretty wacky designs on third-party licensee products (e.g. UNIQLO clothes for a while, a number of unusual licensed products for this anniversary year).
I would wager it's one of two scenarios;
1. UltraPro is being overly cheap. They don't have to commission/pay royalties on top of what they already pay to TPCi to use stock artwork and some generic background.
2. UltraPro has made TPCi some proposals, but they're somehow so out there as to be objectionable.
I always thought UP has been half-hearted in how they handle their licensed products. I'm a little surprised TPCi hasn't already ended the agreement and handled this all themselves, they don't seem to be bringing anything to the table that's particularly difficult to do.