In the past, licence holders were automatically given C1 class along with their B licence. This is no longer the case and now requires a medical from GP, which is why many people have not bothered renewing their C1 licence.
In other cases, some people don't tick the box for the C1 licence and therefore it is not given (still requires a medical however).
However, if you had C1 on your licence within the past 10 years, you can re-apply with the medical cert and have it re-instated.
However, there is still a loophole in the system, whereby you can obtain a C1 licence without going back to basics.