I honestly think that they never thought about it or simply don't care. Also I think the numbers that voted for 6 would have voted for 5 if it were between just 4 and 5. That would then leave 5 as the most voted for. A lot of people have said 4 or 5 works for them.
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Very surprised how many are voting for 4 man rather than 5 for this exact reason.
I don't care about any of the other options except for that one, and having 5 would be ideal for end-game content.