1 million people within less than 30 minutes 4 million people within less than an hour 5.5 million people within less than an hour and a half 7 million people within less than two hours

As you can see from this map Grenoble airport is right in the middle of Rhône-Alpes. In fact it is more central than Lyon airport. Checking travel times will show that all the main cities in the area are at least as accessible from Grenoble airport, if not more so, as from Lyon.

All the towns along the Alps are obviously closer to Grenoble than to Lyon. They account for roughly 50% of Lyon airport’s catchment area and all their air travellers drive past Grenoble airport on their way to Lyon.
Few airports in France, or even Europe, have such a good catchment area, or they have already reached saturation.