This is not something that can be altered; it's part of the Neverwinter Nights engine (and, for that matter, part of Dungeons and Dragons since 2nd edition).
Moreover, Charisma plays a part in Cleric abilities as well (turn undead and similar abilities). Wisdom reflects (to an extent) the ability for a divine caster to connect to their deity, reflected in the number of spells granted by said deity. Charisma represents force of personality, which is reflected in their outward expressions of personal faith (smiting evil, turning undead, etc).