How do you politely tell your friend that he/she is not invited in your wedding when they ask?

I will be frank and explain the reason. Most people understand that there is a limit to the number of friends one can invite after taking into consideration family, relatives, family friends etc. My wedding was a small and low-key affair. Only close friends were invited and I did not get anyone outside the invite list asking about it. Some of my friends have a "backup" list. Perhaps if you are considering inviting that friend (if someone on your main invite list is unable to attend), you could simply tell your friend that you are still working on the invite list and it is not finalized yet?