Extra Credits posted a video about why it is that Fallout Shelter, which seems to almost be the antithesis of what hardcore gamers want with it's F2P and Play to Wait gameplay, is so popular with hardcore gamers. Worth a watch.
Good analysis. While I'm sure no PvP has certainly enticed more hardcore or console gamers to try out Fallout Shelter, I suspect the "loot system" is what keeps many gamers coming back for more (it did for me, anyway). I think the video even misrepresents how easy it is and how frequently one can unlock lunchboxes in-game without paying real money. He also doesn't mention the very rare Fallout 3 characters that you can unlock or the OP weapons like the Fat Man--something FO fans would find particularly appealing. Even the rare higher-stat characters are a lot of fun to unlock for their own sake (not just because of higher stats/power) because they come with a unique costume and look.
Regarding lack of interactivity, there actually is quite a bit to do once you build out your vault to 6+ levels, with dozens of rooms and over 50 dwellers. I know it's interactivity in a management sense, but once I get to that point, I can be continuously occupied for 30-40 minutes, collecting resources, completing objectives for caps or lunchboxes, building and upgrading rooms, rotating dwellers between the production and training rooms, equipping them and selling excess gear to manage inventory, doing various things to increase morale, and of course dealing with or preparing for incidents, like Deathclaws. I actually spend more time "waiting" in traditional AAA action games that have a lot of cut scenes and loading screens.
I would actually still be putting a lot of time into Fallout Shelter if not for the fact that my entry level LG Android tablet can't seem to run the game properly once my vault gets big. I had essentially created a perfect vault with 95% happiness rating and some very rare OP weapons and characters that even Deathclaws didn't stand a chance against until I ran into the same serious lag and crashing issues I had with my first vault. A Fallout game lagging and crashing -- go figure!