Streamlining. It's cheaper and it doesn't' exactly require a lot of creativity. I wouldn't expect story branching and C&C in future BGS games... After all, the aspect most of their fans seem to value the most is the exploration part.
If you wanted a deeper ending... well blame the whiners who didn't want to the game to end after beating it. For an ending to have real impact... the game has to kinda end and real big impact would take years to come to fruition.
Why can't you destroy the minutemen? THEY'RE ALREADY DESTROYED when you start the game. They only exist as a faction if you build them up. Otherwise they are irrelevant. You might as well ask why you can't kill an NPC who always spawns dead. I
It's your own faction, you're their leader. It's basically an excuse for building settlement. Its existence is basically to have something to fall back on if you piss off all three factions in order to allow us to have an ending. It serves the same role as essential companions, just for quest purposes.
Whose effects weren't visible at any point in the game. They said it happened but you ever see it. If you notice when they did NV they kept ending slides but had a closed ending because people would expect to see those effects in an open world game. The OP isn't asking for an ending slide he wants to see what should happen years down the line occur in-game.
You want an "ending slide" Mama Murphy has one more prediction after the MQ
Or to only wipe out one faction. Minnutemen ending lets you keep the RR and BoS alive.
I feel you, though I'm puzzled by the "first time ever" comment. Bethesda has never really given you any real sense of what you accomplished, because in Bethesda games most things happen in isolation. Fallout 3 didn't really have ending slides - or, it did, but your choices were only reflected in very broad strokes, or in specific less-than-significant ways (i.e. did you side with the Enclave or the Brotherhood? Did you sacrifice Sarah, send in Fawkes, or get irradiated yourself? Etc.), and the Elder Scrolls as a series doesn't really reveal much in after main quests.
Your actions don't have a ton of weight in Bethesda games, because for better or worse, Bethesda has always focused on letting you make your own story and giving you a sandbox to play with by creating an incredibly rich world for you to organically discover. This is why some people prefer Fallout: New Vegas - your actions have a lot more impact and weight in that game - whereas others prefer the total freedom of Bethesda's style.
Personally, I like narrative-driven games more than total sandbox games, which is why, despite loving Fallout 3, New Vegas was (and still is) my favorite modern Fallout entry. But there is something to be said for the fantastic vignettes you find woven into the world in Bethesda games. Hence why so many people pine for a game where Bethesda makes the world, and Obsidian writes the story and characters.
This, of course, is primarily because of resource/technical limitations. In both Fallout 2 and New Vegas, there were a staggering number of variables that you could mix in that would dramatically change the game after the end, and it's simply not feasible from a design perspective to create world states that account for every single possible combination of variables.
Would have been cool if they allow us to choose which faction to wipe out though. I could give 2 whales about RR, but I want to keep BoS and Institute.
Sadly, nowhere. I was asking myself the same and thought that perhaps higher level versions of synths will start to carry plasma weaponary ( since it's hinted they're moving to plasma ) but it never happened. Very dissapointing.
They said DLC will be based on feedback, so if it's something enough people want, they may add post ending DLC stuffs for all factions. I would like to see something post-ending.
If you want to know how yoru actions effected the game world pay attention to what you are doing in the game maybe?
Bethesda never does ending slides because they, like most RPGs, already spoon feed you the answers to what happens next in the game itself.