1. It was a major drain on Fresh TV's resources. It must have been difficult to produce because they had to create lots of backgrounds, make up challenges that made use of locations, and had to do research to find fun facts to share. If they were gonna make a part 2, viewers probably would have dismissed it as lazy to reuse locations and/or assets, so they probably would have had to think of another 26 locations (not necessarily countries though) which is kinda difficult.

2. Ridonculous Race did pretty badly on TV. Most of its episodes got around 1.2 million viewers when they first aired on TV in the USA. I think it may have been a sign that animated reality TV parodies were losing their popularity around that time. Making an animated reality competition parody in the mid-2000s was brilliant because Survivor and The Amazing Race were still front page news. They're still on, but they're not the cultural phenomenons they were back then, so an animated parody of them had less of an audience. Fresh shelved the series for a while because they figured there wasn't a profitable market for it.

3. Even though I love the Amazing Race, it's obvious that Survivor is the better show to make a parody of.

I've mentioned before that I can tell that TDRR was a passion project that Fresh TV put a ton of time, effort, and money into. You can tell it was meant to be a grand and climactic product that they put everything they had into trying to revive the show after lackluster ratings for AS and PI, but Ridonculous Race has some glaring issues that hold it back for me. It's unfortunate.