Imo in order to qualify for this tournament while not being the main protagonist, you need to meet at least one of these criteria:
1. Have basically the same amount of screen time, relevance, and plot importance as the real MC (like Killua for most of HxH)
2. Take over as the MC for a major stretch of the story (like Kurapika did in Yorknew and now the Succession War).
I wouldn’t even include Sasuke and Vegeta here tbh, but they have more plot relevance than Zoro.
Ask yourself this, for how many arcs in OP has Zoro been just as important as Luffy or at least nearly as important?