How come syndicated shows do not air all episodes produced?

I find it odd that some episodes are entirely wiped out and not aired. I see this all the time with Home Improvement and Saved by the Bell reruns.How come syndicated shows do not air all episodes produced?
They buy episodes in bundled packages and sometimes dont have the rights to every episode. That happens with really long running shows like the Simpsons, they dont want to purchase the rights to 20 years of episodes, they'll make do with the past 10 years.How come syndicated shows do not air all episodes produced?
i think because some of the episodes were too racy when they first aired and they don`t want kids these days to see them, they think that they cant handle it. they don`t even show that one episode of boy meets world the one were cory and shawn get drunk or the prom episode.