Because it is essentially redhat without the license fee. If you are in enterprise space and all that vendors certify for their software is RHEL, chances are very high it will work well on CentOS.
Entirely legal and ethical under the GPL, and RH benefits from all the folks who know the RHEL system because of exposure to CentOS - when they're recommending a supported version of Linux, it's likely to be RHEL.
It feels a little bit more moral to load CentOS on a bunch of systems, than to load Red Hat on them and only pay for one support contract. As an analogy, Microsoft probably prefers people to pirate Windows than to switch to Linux/Chromebook.
The truth is, Red Hat is the only linux distro I can think of (besides maybe SuSE, but I don't know much about it) that has really made a significant push to take over the enterprise market from Microsoft (and now they've moved to trying to do the same thing to VMWare).
If you want a warranty/support contract, definitely purchase RHEL. If you don't need that, but want enterprise grade stability and compatibility, use CentOS. If you want to run Linux on your laptop, don't use either.
Actually CentOS 6.4 has been the most reliable thing on my Lenovo T61, T400 and Dell T3500. Everything works from hibernate, sleep, WiFi, SAS, SSD's out of the box with no configuration.
This is the polar opposite of Ubuntu, Debian stable, Debian testing and Mint which are royal bastards to get off the ground.
Never again. Always seemed laggy. I just loaded Scientific Linux 6.3 on 3 old thinkpads this past weekend. As you said, everything works. No "sudo vi /etc/X11/xorg.conf", no modprobe & ndiswrapper. Red Hat was my first peek at linux, so I'm a tad nostalgic (remember 9.1? crux, luxi sans?).
There is a super useful guide for the final polish on a EL6 system (extra repos, Flash, VLC, ..) here: http://astr.tohoku.ac.jp/~akhlaghi/newlinux.html
That's good because given the amount of code Red Hat contribute back to the community, including buying proprietary tools and open sourcing them, you'd look like a complete idiot.