If the point of giving us mitzvot is to allow us to earn a reward in olam haba and to perfect creation (there's all of Jewish philosophy boiled down into 102 characters), why wouldn't all Jewish adults be equally obligated in all mitzvot?
I understand someone who is physically unable to perform certain mitzvot but outside of that it would seem to be unfair to someone to exempt them. I realize that in some circumstances they can still perform the mitzvah even if they are not obligated to do so, but the rule is one who is obligated and performs is greater than one who is not obligated and performs.
So then why not simply obligate all who (can) perform equally?