My best guess: because its targeted at home users. Not many home users have access to greater than 1 GB access. I think my router can link supports 1 GB, but I dont know because I rarely transfer files between my computer (Wife is Mac, I'm PC/Linux). My comcast DL is only about 150 Mbps. My nearly 8 yr old PC actually has 1 GBs, but its basically unused.
at 4000+$, yea, it's definitely not for home users
and those few ethusiats that will have this at their home will definitely have 10Gbit
also your 8 year old facebook machine might be ok with 1Gbit but definitely not this 24-64 core beast, people who use these will definitely benefit from 10Gb
it's also not related to internet use as much as connecting it to storage on local network
edit: with the 64 core i'd go as far as saying that it should have a 40Gbit option as well
