The timestream is, generally, very stable. Like a river running downhill to meet the ocean, time has a tendency to correct its course towards a harmonious path, even if changes are made to it from an external source. This is why time travel is considered impossible: time would just revert any changes made to maintain its consistency.
In order to change our world and the direction humanity would take, it was necessary to shift the entire timestream to a completely new balance, a new harmony. As a result, things that once happened and affected the way the world works today, didn’t happen at all, and new events were able to take place that before didn’t happen
The difference between what once happened and what is in line with the adjusted timestream, is what is popularly being called the Mandela effect.