The mishna, M'gila 23 amud 2 as described by Rashi, says we don't say the mourners' outdoor blessing without the presence of ten people, and the mourners themselves don't count toward the requisite ten. Rashi explains that the mourners aren't included in the count
because he's saying a blessing for the consolers separately ("Our brothers, may the master of payback pay you your good recompense. Blessed are you, God, who pays back.") and for the mourners separately ("Our brothers, may the master of consolations console you. Blessed are you, God, who consoles mourners.") and doesn't combine them.
If that's the reason we need ten non-mourners then we should need ten mourners, too. Why don't we? Or do we? Or am I missing something?