When Is Automated Not Automated? (Updated)

Last Friday, I submitted a transfer of funds from my bank account to my Prosper.com account and was informed it would take up to 5 business days for the transfer to be completed. Huh? The process is automated! In fact, Prosper uses the ACH system for transferring funds and ACH stands for Automated Clearing House. Last time I checked, computers didn't sleep or need time off, so why does this system only work during business days? Does anyone have the answer?

I can use my ATM card to make a purchase at a store on a Sunday, then go home and log on to my bank's website and see the transaction posted immediately. So why can't this happen with transfers using ACH?

Of course, the obvious answer is that ACH is a different network than the one debit cards use. But this simply begs the question of why Prosper choose to use the slower ACH system rather than the faster system debit cards use...

Does anyone have any insight into this?

Update: I contacted Mary Zerrenner of Electronic Payments Network, the only private ACH operator in this country, for answers to these questions. She says the settlements for ACH items happen at the Fed (federal reserve bank). The Fed does not process settlements on Saturdays, Sundays, or holidays. Thanks to Mary for looking into this. However, the underlying question still remains: Why are the computer programs at the Fed not running on weekends and holidays?