... Because btrfs is far too immature to actually live up to the its potential reliability benefits.
Sure, I'd love to have ZFS-like-features on Linux, but for now, ext4 is more reliable than btrfs.
DEV: Intel S2600C0, 2xE52658V2, 32GB, 4x2TB, GTX780, F20/x86_64, Dell U2711.
SRV: Intel S5520SC, 2xX5680, 36GB, 4x2TB, GTX550, F20/x86_64, Dell U2412..
BACK: Tyan Tempest i5400XT, 2xE5335, 8GB, 3x1.5TB, 9800GTX, F20/x86-64.
LAP: ASUS N56VJ, i7-3630QM, 16GB, 1TB, 635M, F20/x86_64.