Fair question. The most obvious reason is that we don't have a 24" iMac to write the guides with!
Also, writing guides is quite time consuming for us, and getting all of the iMac guides we have took us quite a while. We're taking a break from writing these guides until we see what customer demand is for the parts we've already got. The best way to get us to write more guides is to buy parts!
That said, we do sell the part you need. You can use [http://www.ifixit.com/Apple-Parts/1-TB-7200-RPM-SATA-Desktop-Hard-Drive/IF107-083|this drive] just fine.
We'd love it if anyone out there wants to write manuals for this device. Let me know if you're interested. Going forward, we'll be enabling people to add more and more repair content to augment ours.