What's the big deal? It's a yes or no question: can you come or not? Send an email. I'm assuming if you haven't RSVP'd by now, you aren't coming, but would it kill you to let me know? How can I plan? For chrissake's, come on already.
You don't have to apologize for your lateness, you don't have to give an excuse. A simple yes or no - I will be there or I cannot attend - will suffice.
People who give parties regularly tell me this is always the case, that it's like pulling teeth to get people to RSVP. Geez.