UA doesn't sell gift cards, and there was a ~4 year gap between closing the Gift Registry and opening the TravelBank for contributions. There could be a high rate of credit card fraud on these transactions which might make it more trouble than it's worth to operate.
That's a good point, although I'm not sure why the fraud rate would be any higher than it is for other types of transactions (like ticket purchases). The funds don't go into some anonymous instrument like a gift card, but are purchased directly for the benefit of someone. So if there's fraud, it's pretty obvious who had the motivation to make the fraudulent transaction.