becuase everyone doesnt have a motherboard capable of 166FSB!! think about people with the N-Force, a fairly new motherboard, yet the Athlon would suddenly become incompatible with it!! The N-Force for example has a 266 DDR limit on its chipset, another example is the KR7A, a fairly new and overclockable board, but the natural top end of the FSB is 133, or 266 DDR. It just wouldnt work, not for a while any way.
This is much like the change from Athlon Bs to Athlon Cs, (100 - 133 FSB increase). Everybody with the old KT7 which only naturally supported 100MHz FSB wasn't very happy! This is probably one of the reasons AMD never took the Duron to 133. To give customers with old boards at least a chance to upgrade.
AthlonXP 2500 Barton
MSI K7N2 Delta (MS-6570)
Nvidia Geforce FX 5600 128MB VIVO
512MB (2x 256MB) Crucial PC2700
2x 120GB Seagate Barracuda 7200.7 8MB
52x/24x/52x Lite-On CD-ReWriter
550 watt Q-Tech PSU
Superflower silver case
T1 Internet Connection
AOC 17" 7Klr