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It's all about expected disk sizes in 10 years time (when RHEL 7 main support ends). Ext4 scales up to around 16 TB -- I know it theoretically can support a much larger volume size, but in practice it doesn't work so well. XFS handles tens of terabytes without a problem and Red Hat has been supporting huge XFS volumes for years with many customers (who in earlier versions of RHEL paid extra for that support).

http://lwn.net/Articles/573690/ Dave is a redhat employee.

Dave Chinner's talk is great: http://www.youtube.com/watch?v=FegjLbCnoBw

Their gluster server offering also uses XFS, so probably for support reasons, they saw XFS had better long term support for both sets of users.