Do other factors affect the chance or is it just luck?
I think it is only affected by "luck". There is a random number (generated on registration?) for everyone. In order to know it, you must keep on posting for a few months. Well you "are guaranteed to become Legendary somewhere between 775 and 1030 activity". So 1030/7 = 147.1 weeks. So it is you are guaranteed to become a Legendary after 147.1 weeks of posting.
But Legendary didn't exist for years.. so how did those people that registered before Legendary existed get Legendary then? it cant be registration i think (or maybe it was an unused number or something)