To whomever made the point, to say that third parties all did poorly on the GCN is absurd, as the GameCube was the console Sega was most consistently successful on during that generation, 2K games aside. Sonic's countless outings, Super Monkey Ball, Crazy Taxi, Phantasy Star Online and so on all did respectably. Super Monkey Ball (and its sequel) clocked well over a million a year before Billy Hatcher came out. The game flopped because it was a boy in a chicken suit, the marketing was bad, and the game was mediocre. It was half-assed at best, and it is only looked back at, even tepidly, because Sonic Team hasn't got a creative bone left in its body and Billy Hatcher is, I believe, their last stab at something remotely unique.
Burning Rangers wouldn't have been too big a project. It came out the same year Sonic Adventure did, and I think we can all assume which of those projects got the lion's share of Sonic Team's attention. Moreover, Sonic Team would have been developing Samba de Amigo and Chu Chu Rocket around that time, and presumably dicking around with Air NiGHTS.
I think you're missing the point with Burning Rangers; it wasn't a success, even in the land where the Saturn was successful. If it had been a success, even if it was just Japan, it would have had a sequel. Success in just Japan did mean sequels, because damn near any franchise that started on the Saturn was only successful in Japan - Panzer Dragoon, Virtua Fighter (still seeing iterations today), Sakura Taisen, Virtual On, and so on weren't exactly blockbusters in the West, but they were still given multiple sequels across several generations. Even Panzer Dragoon - and I suspect that Panzer Dragoon Saga had a substantially larger budget than Burning Rangers - managed to squeak out another title after the Saturn.
Burning Rangers, as far as I can infer, wasn't a big budget project; it was just one of many things Sonic Team had juggling. Even in that context, it sold very poorly, even for a 1998 Saturn game.