SNCF stations et Connexions has chosen to offer Wifi connection mainly in the areas where customers spend the most time, while they are waiting for their train. The Wifi coverage is progressively being extended to the crossover platforms in the main stations. However, investing in network coverage for all platforms cannot be justified as concerns customer expectations, especially as the Wifi is provided on-board some trains.
Remember that the Wifi is free in the station and offers a signal strength that is constantly being improved.