Because it only has one "lose" scenario which makes it the safest

That describes what beats it (which the table gives us the information) but it doesn't explain the reason for it.

I'm asking in a game theory / balance scenario, why do we have this particular option.

So far, all options have equal number of "win" and "lose" scenarios except:

1) Vertical attack is 4 wins and 3 loses (+1 net)

2) Backdash is 1 win and 2 loses (-1 net)

From this we infer that if we combine Guard (neutral number of wins/loses) and Backdash (-1 net) we can actually end up in a neutral position (because both those options already lose to vertical B). At the same time, this will reduce vertical attacks to 3 wins and 3 loses (because you no longer double count a win against both Guard and Backdash options.