Because it was a dumb rule.
Mark Gruenwald in particular was a big proponent of this concept, because he liked things to be orderly, and he spent a lot of years hammering nails into the idea that this was how time-travel worked in the MU—invalidating several earlier stories in order to do so in some cases.
I on the other hand have now spend an equal or greater number of years prying all of those nails back out again. Because if any time-travel story just results in an alternate timeline, then no time-travel story has any stakes or any drama. It’s neat and clean, yes, but also bloodless. Time-travel should be dangerous, it should be risky, and there should be consequences, potential or otherwise. At least that’s my feeling.
So, my assumption is that there are several different methods of time-travel that exist within the MU. Some of them create alternate timelines, and some of them do not.
