Because the election wasn’t free or fair. There are lots of good candidates in prison and exile who were not allowed to run, MCM was barred too.
I just have no faith that anyone can create the conditions for it now, Maduro would probably arrest anyone signing up to run at this point. What would Lula say then? In fact, nothing stops Maduro doing exacty what he did before: block observers, intimidate vote counters, obfuscate results.
The ideal would be if the new president from the opposition oversaw the new free and fair elections :/