really imho its one of those fad things, really just jumping onto the wagon of grain free cat & dog food, which is totally different. rabbits don't seem to have any issue with digesting grains compared to that of a dog or a cat. I don't really think there is any evidence for grains causing harm, or a lack of causing harm.
I do remember back along being told that the only reason they created it was because people kept asking for it, otherwise they had no intention too.
mostly in comes down to ingredients, the grain free has a lot more ingredients in it to make up for a lack grains in it. I can't comment on whether they are bad or good as a whole, but I always prefer simple over more. I expect these ingredients are purely just to bulk out and bind the pellet, adding little nutritional value, but I could be wrong.
grain free: Timothy hay, soya bean hulls, pea flour, flaked peas, ground soya bean meal, ground dried locust beans, whole brown linseeds, soya bean oil, calcium carbonate, hydrolysed yeast, Yucca extract.
normal: Alfalfa meal, soybean hulls, wheat, wheat feed, flaked peas, linseed, beet pulp, soybean meal, soya oil, monocalcium phosphate, salt, calcium carbonate.
the grain free is not any higher in fibre, in fact lower. its also not any higher in calcium.
grain free: Crude Protein 14.0%, Crude Fibre 22.0%, Crude Oils and Fats 4.0%, Crude Ash 5.0%, Calcium 0.6%, Phosphorus 0.4%.
normal: Crude protein 14.0%, Crude fibre 25.0%, Crude oils and fats 4.0%, Crude ash 8.0%, Calcium 0.6%, Sodium 0.3%, Phosphorus 0.4%.
really, there isn't a lot of difference other than slightly lower crude fibre, and more ingredients to bulk out where the wheat is lost.
unless there are multiple grain free foods which in that case I'm sorry
https://supremepetfoods.com/products/grain-free-rabbit-food/
https://supremepetfoods.com/products/science-selective-rabbit/