If you ask 30 members that question you will probably get 30 distinct answers with different reasons.
I have 6 rabbits and one is a mini rex, so I will say what works for me. They get 1/4 cup oxbow bunny basics pellets in the morning, 2 c leafy greens etc at night. The etc. is made up of herbs, flowers, other vegetables, weeds, small amounts of fruits, the occasional treat such as a cherio for extra good behavior. Unlimited timothy and orchard hays and just a hand full of alfalfa hay. Ofcourse clean fresh water at all times, I prefer a bowl because I think they drink more. I use the idea of having a varied fresh food diet helps cover all the bases and keeps them from being picky eaters, but none of mine have had issues with this choice.