Different cooking methods produce different results, it’s not just about onions. But the maillard reaction comes from frying them, theres also a textural element to the difference. Using fat promotes allows browing and even caramelization.
Try using less fat to fry them instead of 0 fat. If you’re talking about deep frying, then try the microwave method from americas test kitchen.
(Im not a 100% sure I’m right but i’m fairly confident)