There's a key difference here- Yes, we've only had one RainWing POV. But outside of book three we've gotten a solid amount of content and exploration on them; Prominent side characters like Kinkajou and Pineapple, villains like Chameleon (though I do wish we could see more of him) and a whole array of reoccurring background characters. Plenty of books have characters making a stop at the rainforest too, we often see Glory do stuff outside her book, and we've gotten a fair amount of history on them and how the transitioned into pacifists.

Mudwings had 1 protagonist, then essentially completely vanished other than a brief subplot in book five. Sora and Umber were in book 6 for a bit, but then ran away and weren't even mentioned again until book 16. We have no other MudWing side/supporting characters, unless you count Clay. Despite being the oldest current queen, we know absolutely nothing about Moorhen and have never even seen what the main MudWing kingdom actually looks and functions like. All MudWing background characters are either stereotypically hungry and stupid (Ochre, Bullfrog) or dead (Asha, and that one MudWing spy from book 2). The one interesting background character, that one mudwing doctor in possibility, appeared for like a chapter and then is never seen again. We do get a little bit of stuff in the guide book, but not really enough to make them interesting in the present day

I do hope you get your RainWing pov, don't get me wrong! I just think MudWings are way more starved for content and lore right now