I think it's mostly that even "smallish" power draws like 300W adds up in cost quicker than you think if it's on 24/7. I don't know where you live or what you pay for power, but I live in one of the cheapest places for power in the US and pay ~$0.12/kWh, and that still equates to ~$1000/year/kW for 24/7 operation. I know when I finally did the math it surprised me.

There's tons of cheap used or new-old-stock 1.2TB 2.5" SAS drives on eBay. The 1.8TB drives are significantly more expensive though.