So contrary to how people use the term, a “litter box trained” rabbit isn’t a rabbit who recognizes a litterbox and goes there willingly– it usually is more the overall concept that the owner has set-up the rabbit’s environment in a way that aligns with where that rabbit naturally wants to go. Starting with a smaller sized area can help rabbits learn that their litterbox is a safe place to eat/go poo/pee, but ultimately making sure you (the owner!) are making the litterboxes spacious, appealing, and in the right locations are going to maximize your rabbit’s litterbox adherence.

https://wabbitwiki.com/wiki/Litter_training

It would be good to keep in mind the environment that he came from. He was likely used for breeding and for showing. He wasn’t a house rabbit. I can’t say for sure, but he may have been housed in a cage with a wire bottom. This is a common setup for breeders. He may have had a litter box in his cage, but he was probably used to just pooping/peeing wherever because it would simply “disappear”.

Bunnies vary in their desire to do their business in a litter box. My bunny took to the litter box immediately. I didn’t have to do much to persuade him. He wanted that to be his spot. I can move his litter box and it doesn’t really matter to him. He wants to go in that box. Many bunnies are not so easy though. They decide where they want to go and, as Wick said, their owners simply take advantage of that. They see their bunny prefers this corner so they place the litter box there. They make the litter box inviting by having soft bedding and lots of yummy hay. It helps to put any “accidents” in the box to reinforce the litter box as the place to go. But ultimately, some bunnies just won’t be as “litter trained” as others. I think a long history of not using a litter box could certainly contribute to this, but not always. Neutering does help. I assume he isn’t neutered if he was a show rabbit.