Many companies are, let's say a bit lazy - when use an Active Directory domain anyway, you might as well use the DHCP and DNS servers, too, they handle replication and failover very smoothly. (I am not a big fan of Windows, but that part has worked pretty well in my experience.)

You can get a similar mechanism to work between BIND and ISC DHCPD; it's not a lot of work, but with Zeroconf/mDNS it is less useful than it used to be.

Colocation means many clients, and in any given colo there's almost certainly someone running a Windows AD + Microsoft DHCP box, meaning it's "in" that datacenter. I'm surprised as many as 40% of networks still have that tech, but that's enterprise for you. Point being, though, it's likely in well more than 40% of datacenters.