The same reason we need to daven in a fixed place, makom kevua. To daven is like an arrow which when shot always from the same place, will penetrate. So the expectation of a gate for a particular shevat is a fixed prayer. When we daven betzibur, in a group together, it should not matter. The custom of a person might override the nusach of the tzibur according to the Chasam Sofer above.