When you burn a CD why does it stop half way through?

There are a number of causes. It could be that the CD/DVD burner is defective. Or the media itself is defective.

Some of the early CD burners had firmware bugs, so they would burn CDs that they could read, but that regular CD players couldn't read properly. So the CD player stopping on a burned CD before reaching the end is a possible symptom that the CD burner doesn't produce standard CDs. In that case, you might be able to get a firmware upgrade for the burner from the manufacturer.

Or the operating system is messed up. For instance, the operating system might be operating the CD/DVD burner in PIO mode. You would have to search in a search engine to find a tool or patch to restore the ultra DMA modes. If Windows has everything in PIO mode, it could be due to using bad CD/DVD media, or you could have other problems such as a hard drive about to fail.