Has anyone made any research on this topic?
Maybe the reason is lack of trust to the recently installed app or bad reputation of pushes?
Push notifications are a great way to engage app users, as long as it's not annoying. One example of this is wayfinding apps with beacon technology that detect when mobile app users are nearby and send push notifications to let them know something important. In this case, push notifications could alert a user of a special sale, which can be perceived as spammy, or that they have arrived at a certain desired waypoint (more useful, and less spammy), along with interesting information about that location. Think of how useful these are for tourist destinations, like museums, zoos, etc.
You've downloaded xyz zoo app, walk near the big cat area and get a push notice, "grrrrowl, don't miss our feeding time. In 30 minutes you can see us chowing down on fresh antelope quarters".
Because it is annoying. Many people have various apps installed. If they enabled push for every app that would basically mean their phone ringing / vibrating almost every hour which is really annoying. They only enable push notification from important apps that are crucial for them such as Whatsapp or Twitter.