More like probably 6th or 7th? Where do you get this second biggest from? Networking is cut throat, you have to fight tooth and nails to get market share. Arista from a tech standpoint may look good compared to Cisco - I mean shit, how hard is it NOT to look good against a 10+ year old switching platform.
Obviously the people that are using the 6500 1) don't care about costs 2) don't care about performance(they'll just buy more) 3) don't care about power efficiency/usability/etc 4) don't want to invest in anything new(e.g. change) - they are happy with the old way of doing things.
I remember reading up on marketing stuff way back in 2004 on chassis switching platforms that destroyed the 6500 in almost every way (except sales of course). I'm sure there was stuff earlier but that was about the time I started getting more involved in networking.
Just looking at when Arista was founded- Oct 2004, these PDFs I have came out before Arista was even founded(and obviously years before they shipped any product)
Maybe those previously mentioned cloud players(another article) that are "shunning Cisco" were dumb enough to do wide scale deployments of 6500 and that's why their costs are so high.