I suppose you could continue and ask why even 71 should be enough - we should require an unlimited number of jurors. But that would of course be impractical. So the Torah makes a compromise between the value of a human life and practicality.

23 judges is really 10 judges. (That is, ten vote to kill the person against another ten who vote to leave him - the other two are needed for giving the former a "Rov" - as the Mishnah explains.) A quorum of ten is like a "full" person; that is, it represents the range of aspects (or mindsets) of a single person. It is the amount required for Devarim shebi'Kedushah, because the Shechinah is there. By offering the convict a jury of 23, we are allowing the expression of every way a person thinks to be used in order to save him.

71 is only necessary for infractions that pertain to a Tzibur (community), and not just a single person (or even group of individuals). It represents all of the possible ways of thinking in a community. (See Ramban Bamidbar 11:16.) This number is used to judge the Tzibur (as in the case of Ir ha'Nidachas), or a person who embodies the Tzibur (such as a king, Kohen Gadol, or prophet).

Best wishes,

Mordecai Kornfeld

Kollel Iyun Hadaf.