The reason .io domains are so valuable is because companies that use .io domains are known as being great at what they do.
"These new .io sites are almost artisanal in nature - small, well crafted and functional. It's like a TLD for techies with taste."
They are also short, sound good, and .io domains perform well in Google's search results. The reason for this is because they've been added to the geotargetable domains list.
Microsoft bought Doc.io for $24,600, Elasticsearch just bought es.io for $30,000, cloud.io sold for $45,000 on Flippa, music.io sold on Flippa for $20,000, traffic.io on Flippa for $14,000, and fd.io, db.io, sports.io, mega.io, groups.io, leaf.io, mine.io, and many more recently sold for $10k+..
A lot of the tech startups that use .io domains are now very successful businesses, further reinforcing the idea that companies that use .io domains are "in the know" and part of the sophisticated tech crowd.
Check out just some of these examples:
Iguaz.io lands $15M Series A
Tuition.io and Test.io each raised $5M
Google acquired spider.io
Easel.io was acquired by github last year
Keen IO just raised $11.3 million
Nitrous.io just closed a $6.65 million Series A round
Ginger.io raised $6.5 million
Tuition.io raised $6 million and manages over $1 billion in student loans
Intercom raised $6 million
Greenhouse raise $2.7 million
kontakt.io raised $2 million
Luma.io acquired by Instagram
Rackspace acquires Exceptional.io
And many more
.io domains are increasingly popular - many startups prefer .io over .com And they are more popular than .org as reported by Google Trends.