Because of too many simultaneous downloads the server keeps connection on wait while switching other users connected.
Can be explained in this way - Just like not everyone is successful in getting SIM and those who managed are also waiting for activation because of over rush.
Same goes for apps also - MyJio is at the top in Google Play Store, though it is only being downloaded in India and that too only by 4G handset users who want Jio Sim.
This reflects the traffic load on Play Store server - leave apart other apps.