Here are the two major reasons why:
1. I was called away for 14 minutes. I know this as I looked at the time when I left and the time I returned. When I returned, for the second time this week since I installed the 1.2.1 update just over a week ago, my vault was in shambles.
1a: 10% happiness ratio
1b: Approximately 70% of my vault was without power
1c: Every vault denizen had less than 50% health
1d: Every vault denizen was suffering from 40 to 50% radiation poison
1e: Even though radiation poison was removed from a denizen and healed, the denizen continued to suffer from radiation poisoning from an unknown source
1f: Happiness ratio is improving at a snails pace despite removing radiation poisoning/healing denizens. At this current time 32 minutes have passed and the happiness ratio has rose from 10% to 11%; or, 1%
1g: To top things off, the game decides to send two Deathclaw attacks at me killing 71 (63%) vault denizens of which I am unable to revive due to previously spending caps to increase power output, upgrading two food production rooms and building two new stimpack and radaway production labs; which, btw, are currently without power. And, at this time, I'm now I'm back down to 10% happiness ratio...
2. Zooming in to raider, Deathclaw, fires, molerat infestations, etc. This is the second most irritable issue I have with Fallout Shelter. I do not want my view zoomed in to the any of events listed previously as it delays my ability to address the event in a timely manner. If left zoomed-out I can quickly and easily assign denizens from other rooms to combat the event. As it is, I have to zoom back out and scroll to the closest room to start moving denizens in to deal with the event.
To be very forthright and clear, I do not use hacks, cheats, etc for any game I play. Never have, never will as they break the enjoyment of any game for me.
You know, I loved Fallout Shelter when it was released. Had a lot of fun, I thoroughly enjoyed being able to manage my own vault and actually spent real money on the game. Now, I am very dissatisfied with the events that have taken place in one week. As such, I am submitting a refund request through Google play. I didn't invest the money into my vault only to have major, game breaking glitches as described above take place which have ruined my experience with Fallout Shelter.
Maybe if Bethesda fixes the issues then I'll return. If not, then that is the way it is and I'll leave Fallout Shelter behind as I have many other glitchy games. I have many other games to play, including other Bethesda games, which do not have these issues.