In the old days, Pie was probably the best balanced LoS 6 player map.. so it was only natural it became the most played as well.
Also at that time, the rushing play style was pretty common, something which fits Pie well..
La Corrida is also pretty good, but it can occasionally leave players with only a single 1-bar rare, which is annoying. (Also, with only 1 entrance, it's either noob friendly or lacks diverse strategies - whichever option floats your boat I guess haha)