Unfortunately the novel has been in hiatus since forever, we might never have an idea what happened behind the scene leading to that party composition.
If I have to guess, the first reason is the hero requesting a full female party, which narrowed down the number of candidates a hella lot. Second is from Gregorio and Stei's examples, you can expect that all strong individuals in this world have some screws loose (or else they wouldn't reach that level in the first place), forcefully enlisting them could sabotage the whole operation instead. Heck, the reason Ares got chosen might not because he's strong, but he's the most sensible guy among a bunch of fanatics.