[citation][nom]ramicio[/nom]Where is a 3.5" form factor? The server world still hasn't made 2.5" mainstream because 2.5" drives are for laptops. A 2.5" drive in a 3.5" bay is just a waste of space.[/citation]
3.5" is nearly pointless in the server market - Especially for Solid State Drives. Space and power are reasons for this. If I had the choice, I'd pick 2.5" over 3.5" any day. With Rack-space at a premium, it adds up. The more volume in a server room to cool and house, the more it costs.
When it comes to SSD, space/surface area is not the issue or the limiting factor - the manufacturers limit capacity on cost, not because the 2.5" form factor is too small to fit more silicon, but because no one consumer would be able to purchase a 3TB SSD. Right now, the enterprise level SAN manufacturers (NetApp, ect) are pushing SSD's in the data center and the price - which used to be astronomical for conventional SAS drives - is frigging ridiculous. If the trend keeps going where it is (and I hope it does), then I think perhaps the demand may drive a need for more capacity beyond the form-factor.
By the way, 3.5" SSD's do exist. But you are looking into the Enterprise/Industrial space and SAS is probably all you will find.