Generally, there isn't enough time to convert Sith to the Light on the fields of battle. And they didn't get them all, the Sith Triumvirate commanded the remnants of Revan's Sith Empire.
i thought jedi would always try to convert to lightside first and only kill as a last resort. once defeated, i thought the remaining sith may see that there can be another path in joining the jedi. just surprised the jedi slaughtered them is all.
Had circumstances been different, I'm sure the Jedi would have tried to convert them.