A few reasons actually...
1. It was time for a new gaming rig!!!!!!!
2. I already had an IBM server running win2k advanced server, a PIII tower, and old PS/2 (don't laugh-playtoy-firewall box) and of course my ThinkPad. I had and still have everything on a kvm and audio switch. I just needed the smaller footprint due to all the other systems at my desk.
3. Looking at all the integration with the SFF, I didn't need slots for a NIC, sound, USB 2.0, Firewire. I couldn't think of anything other than a high end video card at the time, and I don't use dialup anymore so I still have 1 PCI slot free. And I can always add stuff via Firewire or USB so it's all good.....
4. Looking at the size and relative cheapness of today's hard drives, I didn't need space for more than 1 drive ( I got the shuttle), and if I upgrade that, I'll just ghost my image onto the new drive.
5. I only needed one optical drive, and I can burn DVD's and CD's all in one so the one 5 1/4 bay is plenty.
6. I bonused the system by adding a card reader in the 3 1/2 bay where the floppy usually is installed.
I like the looks too, but that was my last consideration. Heat was a concern, so I did a lot of trolling of forums and review sites to ensure I was making the right choice.... which I did.
Now I'm thinking of retiring a tower and building another Shuttle SFF.