Lots Of Reasons
There are lots of reasons why a PhD candidate might not get to submission.
When I did my PhD I knew someone who didn’t get on with their supervisor and he left, thankfully to do his PhD elsewhere. Sometimes a person’s funding isn’t secure and they aren’t able to continue. Sometimes people start a PhD and it’s only through doing the work that they realise it’s not what they want to do, so they stop.
For these and many more reasons, some people who start a PhD journey don’t work through to submission and the viva.
There are also lots of reasons why PhD candidates who get to submission go on to succeed at the viva.
They did the work. They have made a contribution to knowledge. They are knowledgeable. They are a capable researcher in their discipline. They learn what to expect from the viva process. They do the necessary work to get ready for their viva.
For these and many, many more reasons, PhD candidates who work to submission then go on to pass their viva.