They're widely used because they were one of the few providers of geo-aware DNS service for a long time. (These days there are other, cheaper options, including Amazon Route53.)
$ dig +short ns amazon.com
ns1.p31.dynect.net. pdns1.ultradns.net. ns4.p31.dynect.net. pdns6.ultradns.co.uk. ns3.p31.dynect.net. ns2.p31.dynect.net.