One, category 6A cables actually cost two and a half times as much as a basic single mode fiber patch cables. Two, cable diameter. Ordinary LC to LC fiber cables with 2 millimeter diameter duplex fiber are much easier to manage then category 6A. Three, choice of network equipment. There is a great deal more equipment that will take ordinary sfp+ 10 gig transceivers, then equipment that has 10 gigabit copper ports. As a medium-sized ISP, I rarely if ever see Copper 10 gigabit.

SFP+ 10Gbase-T 3rd party optics started hitting the market this year, but none of the major switch vendors offer them yet, so the coding effectively lies about what cable type it is. Just an option to keep in your back pocket, as onboard 10Gb is typically 10Gbase-T. Thankfully, onboard SFP+ is becoming more common however.

For short distances of known length, twinax cables which are technically copper can be used. They're thinner than regular cat6a but only about the same as thin 6a and thicker than typical unshielded Duplex fiber patch cables. Twinax can be handy if connecting Arista switches to anything else that restricts 3rd party optics, as Arista only restricts other cable types. Twinax is also the cheapest option.

Latency, power and cost.

The PHY has to do a lot of forward error correction and filtering, so it adds latency (for the FEC), power (for all the DSP) and cost (for the silicon area to do all of the above).

The latency is almost certainly immaterial, 0.3uSec v. 2uSec. That's MICROseconds not MILLIseconds. Power draw used to be an issue, but not anymore.

Consider the power pull from copper v fiber listed here [2]. The Arista 7050TX 128 port pulls 507W while the 7056SX 128 port pulls 235W. Yes, copper is more but we're talking half a kW for 2 TOR switches. And for this you get much cheaper cabling, as the SFP+ are much more expensive (go AOC if you do go fiber, BTW) and you have to worry about clean fiber, etc.

Where fiber is REALLY nice is with the density, although the 28AWG [3] almost makes that moot.

There's few DC builds that can't do end-to-end copper any more, at least for the initial 50 racks.

[1] - http://www.datacenterknowledge.com/archives/2012/11/27/data-... [2] - https://www.arista.com/en/products/7050x-series [3] - https://www.monoprice.com/product?p_id=13510

I figure it's simply the availability of datacenter level switches (and other components).