Time travel was a fickle thing.
In any world, a traveler who meddled with time would be met with unfaithful consequences. Something would always change, almost always for the worst because of one brief change in the past that was made for selfish desires, and that would alter the universe and its occupants for the rest of time until it crumbled to the ground. Which, is why time travelers were basically unheard of in the Wizarding World.
Everyone knew and (mostly all) loathed Tom Riddle. That, however, did not give them a strong enough reason to make a ripple in the galaxy to go back and see what they could have changed to their liking to sew up the hundreds of wounds he had inflicted, because everyone knew how dire that could end. How perhaps he was meant to ruin the world after all, as some sort of punishment sent by a God for the amount of greed that they soaked in.
Regulus, on the other hand, had his entire life torn in half due to Tom. He wasn’t the only one, sure, but he was the only one that brewed with enough hatred until it spilled over the brim and forced him to decide on an ultimatum: Let Tom crush other families and their perfect, dandy little lives, or refuse to reap what you sow and instead see what possible changes could be made. Obviously, Regulus chose the second option. He was a Black, after all.
The 1940s were bland. Full of war, full of corruption and violence, and somehow, worse than the 1970s. It was obvious Regulus did not belong in such an era. It also was impossible to not stare at his mother as a teenage for longer than needed, because she almost looked carefree, devoid of the monster and had warped into, and his father was barely in the picture. He just had to remember his mission and the entire reason he came here, and that was to haunt Tom Riddle and crush the pesky little fuck under his designer shoes.
He just had to hope he wouldn’t get distracted.