Because.. Well, as i recall; it was just a 2 way street. You could blow it up or defuse the bomb.
Ive seen countless articles listing this as their main "it grabbed my attention because of its depth" in fallout 3 but - it was basically the third quest and irregardless of choice, every player made a similar decision.. 50/50.

There was nothing "unique" about it in the slightest from bethesdas point of view. It basically just determined your house for the remainder of the game and wether youd be spending hours on end ridding yourself of -1000 karma or spending hours feeling like you achieved something when in actual fact - you hadnt.
Which, bethesda being kind enough, decided to not force you to make that decision right at the start and instead threw out around 30 settlements you could choose from and build a house at any location in these areas.

As choices go, it lacked subtlety.

The thing is there was no depth to that conflict, there was no fight over resources, ideology, interests or history, there was no history to learn about Tenpenny towers and Megaton, no background or twisted secrets to discover, just a lunatic asking you to destroy it because it obstructed his view...

I agree, really bad example of depth and consequence to point to...

I'm sad bethseda didn't give such a world changing option so early in the game.

In ~40 hours of actual playtime on my main character, I have come across 2 small quests that gave "good" and "evil" ways to complete them. I have still to come across any side quests that completely change the game state. Sad that I can not purge good neighbor as a quest for the BOS causing to no longer have any living people in it. I sad bethseda is very reluctant to give players side quests (not related to the main story) to completely change the world state and how people in the world treat you.

Im glad they didnt.
Fallout 4; you get to know the factions, slowly progress the story and your character to the point where - 200 hours from start you could just finish the story. You invest yourself in them, pick your side. *I WONT SPOIL* then.. Theres 4 options that tend to end in alot of fire in some way..
Fallout 3; you walk into a town, the sherrif randomly trusts you to defuse an atomic bomb, you either blow it up or defuse it, you get a house, your father mentions it once and thats that.

I honestly have no idea. People in this forum defend Bethesda to death with the Megaton example as well.

The quest had two choices, neither of which mattered, and there was zero consequence in it. Whether you blew it up or not, both Tennpenny tower and Megaton had similar meaningless NPCs, and even if you did blow up Megaton they kept Moira alive so you wouldn't need to face that consequence!

Even your father later in the game slaps you on the wrist and that's it.

Megaton is a textbook example on what not to do. It was implied to be a huge deal (when is blowing a nuke not?), but there was no build up for it to speak of, the initiation for blowing it up was borderline imbecillic ("Hey strangesr, I trust you. Wanna blow some [censored] up here?"), the task was childishly easy, and consequences were laughable and the same no matter which way you did it. What with daddy waving a "son I'm disappoint, now lets purify some water the hardest way possible" finger. And the whole thing was done pretty much for [censored] and giggles and to show off some fireworks.