Because WFA and UKF lack strategical flexibility of EFA and for the most part are just one trick meta ponies, you can try something else, but its usually ineffective.
EFA on the contrary, you have multiple BOs and valid strats, these factions are also harder to play then WFA and UKF because of power creep, so they need more strats depending on opponent and map, while WFA and UKF can execute the same 2 things everywhere.
No, its because the guide community is dead not because the factions are shallow.