Because you said so.
It's one thing to have rotating links or headlines, quite another to 'rotate' large chunks of content.
Think about it; if you have five chunks of content rotating, then one is seen (by users and SEs), while four are hidden (from users and SEs).
Not rocket science, and a disservice to BOTH visitors and SEs.
I did not, BTW, and would not, ever, suggest doing something for SEs rather than visitors; I merely pointed out that what you were proposing would harm your standing in SEs, - as they would not see four fifths of your content - I took it as obvious that neither would your human visitors.