That's because the modules are so expensive! A loaded Nexus 7010 is intimidating enough with it's $473,000 list price, but that's without any optic modules...
If we want to link those interfaces to something with 10GBASE SR modules then triple the budget because the optics cost twice as much as the switch:
$1495 / module * 8 blades * 48 links / blade * 2 modules / link = $1,148,160
By comparison, purchasing the same amount of links in 5m Twinax form comes in under $100,000.
So, there tend to be a lot of TwinAx and FET modules in my designs, and the equipment is located carefully to ensure that ~$200 TwinAx links can be used rather than ~$3,000 fiber links.
The tradeoff comes when you put a lot of 5m Twinax cables into one place and quickly find they're not much fun to work with because they're not bendy and because they're thick. That's why I'm so interested in something I noticed yesterday on Cisco's 10GBASE SFP+ Modules Data Sheet:
No word on pricing yet, but "cost-effective" sounds promising since they're positioning this cable directly alongside the Twinax option. Also no word on support by various devices. My hope is that systems which currently support only passive TwinAx cables will be able to use the longer 7m and 10m AOCs.
Also new to the data sheet are some 1.5m, 2m and 2.5m TwinAx cables. Nothing groundbreaking here, but these will be nice options to have. It's annoying to deal with a 3m cable when 1m won't quite reach from one rack to the next.