It's been explained as best we could in multiple other threads. We're talking about software deployment so general software deployment practices apply. There are two main branches of code at least, the "normal" code and the FSD beta code. That means there are at least two parallel sets of versions that will be deployed to different people.
In general, you never want to send a new software version to everyone at once. What if you made a mistake and it bricks the cars or otherwise makes them undriveable? What you do is you send the update to some people, monitor how things go. If it goes well, you update more people and you continue like that in batches. If something is found wrong, you make fixes and you might release a new version which will go through the same circus. You might have a few at the same time to monitor what's going on.
You might have sent a version broadly to everyone but eventually release a new version. The previous cycle took so much time that some people never got it.
There are different countries with different AI needs (we don't all have the same road panels and markings...), there are different car models etc. Tesla may choose to target only some countries or models in some software versions.
You cannot let everyone download multiple GB at the same time from your servers otherwise they'll jam, or you'll need to pay a lot more in IT infrastructure for no good reasons.
These are a few explanations.
