[Posterity note: Not really sure why I wrote this. I think someone asked me to sum up Marvel's M-Day and Civil War arcs, and why they were both big deals. Sidenote to old self: Professor X gets killed by Cyclops, Bishop becomes a deranged time-travelling homicidal maniac, and Dazzler, Dazzler becomes a S.H.I.E.L.D. agent.
Keep in mind, the idea of a "registration act," whether for mutants or costumed heroes was not new, to the day that it was enacted. Due to the efforts of folks like Captain America, Beast, Daredevil, She-Hulk, Professor-X and others, it was largely through legitimate lobbying that it was never made a reality until the Stamford Incident.
Think about it. Every future forecasted in nearly every Marvel line consisted of some form of segregation, governmental regulation, identification or control of the mutant population. Usually involved those loveable, big purple robots. It was going to happen. The question was always of when, how, and is it preventable? Recent issues of X-Factor still hint at the same (or similar) hopeless future Bishop time-traveled from, back in the 90's. But for reasons mentioned later, mutants face an even greater threat of genocide, from an extinction due to a failure to reproduce mutant children, a product of M-Day, described later.
The X-Men were never in any rush to "fight the future," or to proactively prevent the future Bishop foresaw (let alone the Age of Apocalypse.) They had too many imminent threats to deal with. Proteus is back. The Black Queen is rising the dead. People are dying. Right now. So, the most that ever came of the distant threat of a totalitarian future was of Xavier and Beast stressing the importance of maintaining an open dialogue with world leaders. Fair enough. Worked for a time. But after the Stamford Incident, any amount of goodwill earned on the part of mutants was thrown out the window.
Before I continue on with what lead to the Registration Act and Dark Reign, I'll briefly describe a few choice events that happened prior to the Act:
I'm not going to bother explaining the entire House of M arc. All you have to know is that M-Day/Decimation resulted in 99% of all mutants around the world losing their powers. That meant some became normal humans. Some died, because their powers were so intertwined with their physiology that their bodies couldn't sustain themselves. The remaining ~1% found themselves being hunted by anti-mutant groups. Think of those groups from the X-Men Animated Series that ran around calling people "mutie" and chasing the Morlocks around with bats. Except far more organized. And after the Stamford Incident (described below) absolutely no one has any sympathy for mutants anymore. Many of the remaining mutants, particularly younger mutants and those less able to fend for themselves or 198, to be precise, took shelter at the X-Mansion to later establish Utopia, a mutant island nation built on the remnants of Asteroid M, Magneto's former stronghold.
If you want to learn more about that business, look up "The 198" and "Nation-X" arcs in Wikipedia. In any case, all you need to retain from this, is that the mutant population went from growing to dwindling in one arc. Mutants are now in a different political reality, where once, the world was forced to accept them, now, they're forced to protect themselves and their interests from the rest of the world.
The New Warriors (who?) get into a rumble with b-list villain Nitro (and some other b-listers) in a battle in Stamford, CT. Nitro's primary ability is to self-detonate (and subsequently re-form himself.) During the battle, Namorita throws Nitro into a school bus, followed by some taunting. He explodes, except this time, he kills all of the New Warriors (except Speedball, and later discovered, Night Thrasher) and all of his villain cohorts ... and another 600 (or-so) unfortunate citizens, including 60 children at a nearby school. OOPS! Fun fact: The New Warriors were filming a reality TV show while this happened. How embarrassing!
So, as I started out saying, the idea and impetus for a registration act was always in the wings. But the two caveats were always that 1) trying to somehow "cattle" 30 million mutants would be pretty difficult, big purple robots aside and 2) superheroes were just that --- the few mistakes they made in the past were largely made up for by the hundreds of thousands of lives saved. Those caveats were decimated (HA!) as soon as 1) the mutant population became 1% of what it once was, and 2) 600+ people died as a result of a superhero battle, and in the public eye, as a result of "superheroes."