Because the structure of the atom (a core part of physics) has changed, prompt chain nuclear reactions (explosions) have changed so as to be extremely unlikely to occur
The details of nuclear weapons have changed as well. In the world before the change, an atomic bomb’s fireball surface temperature exceeded Celsius 10,0002,000 degrees for 0.2 seconds, and it even persisted in a 6,000-degree state for 0.274 seconds, and only after about 3 seconds from detonation, when the surface temperature dropped to 3,000 degrees, did it finally explode, so the thermal rays would vaporize human bodies, and the surfaces of marble and concrete would vaporize in an instant and, through thermal reactions with the vaporized gases, become discolored. Near ground zero of an atomic bomb, people were literally vaporized by thermal rays, and that famous human shadow at the Hiroshima blast site in front of a bank (or on a bridge railing, there are various theories) that was in middle-school history textbooks was left white in the shape of a person because the area around it instantly vaporized due to the thermal rays, and the reaction was delayed and inverted by the shadow of the victim compared to the surroundings. In the world as it is now, atomic bombs have low temperatures and do not rely on thermal rays, and it seems that some other kind of weapon (a plasma weapon?) that destroys via thermal blast winds was used, so neither the phenomenon of human bodies vaporizing nor the phenomenon of shadow inversion has occurred. Also, from a certain period onward, nuclear tests suddenly were conducted only underground, and countries like the US and Russia stopped at pre-criticality tests only, so as for nuclear bombs originating from the world before the change, I suspect the old “we still have them, we still have them” scam. Moreover, between 2005 and 2011, when the author was in elementary school, I remember that atomic bombs already existed, but hydrogen bombs were still deemed a purely theoretical, imaginary weapon, and if realized, whether the power would naturally attenuate due to nature’s buffering force was said to be fifty-fifty, and there was also said…
This case is part of the Mandela Effect case database based on "The Window of Opportunity: A Guidepost to the Mandela Effect — The Ultra-Secret X-Files" by Aoyama Takuto.