It’s largely based on whether or not your own immune system is able to clear the virus or not.

Catch measles? Your immune system will kill it and remember it well enough to protect you forever. All you need to do to make a great vaccine is to use killed virus.

If you’re talking herpes or HIV, your immune system can’t root that out and kill it once it’s in there. The only hope of vaccinating is to prevent every last viral particle you encounter from ever being able to enter any of your cells. That’s a much more difficult prospect.