Because you are not speaking about Eleni; you are speaking to Eleni.
We don't use the definite article when we're speaking directly to someone and calling them by name. Instead, we use the vocative case of their name without any article. (For feminine names, the vocative is the same as the nominative.)
The dictionary hint for “Sorry” only gives "σιγγνώμη", which is not accepted as a correct answer.
I can't see the mis-spelled hint σιγγνώμη -- are you sure it was there?
I can only see the correctly spelled συγγνώμη. And that is also accepted as an alternative.
λυπάμαι was not present in the hints but I've added it.