The most common reason people lose their citizenship is that they take on a second citizenship and the first country doesn't allow dual citizenship. This means that citizenship limbo exists where you have legally lost your citizenship but can still benefit from it until the first country finds out. Some countries forbid dual citizenship but don't enforce it so people end up in this limbo for the rest of their lives.

There's also a large number of stateless people who don't have citizenship at all.

There's also people who are not technically stateless, but cannot return to their country or obtain any identity documents from it.