Han, the reason, I would guess, is because until now, AMD didn't have enough support from the public to fund their R&D department to look into creating dual CPU setups. AMD's K6 line didn't help them much, but sure got everyone's attention. They created a low-cost CPU that was very effective in the low-budget ranged PCs. AMD made it possible for many people to have computers because until then, Intel owned a great share of the marketplace. Because of this, Intel was able to keep the prices high, until AMD stepped in. Everyone knows competition equals lowered prices. They offered a lower-priced CPU that wasn't up to clock-for-clock as a Pentium 1, 2, or 3, but it functioned, right? AMD is the underdog of the CPU market and they're here to stay a long time. The only thing that Intel has running for them right now is their hopes for DDR and the P4 combination as well as their brand name to sell their products. Otherwise, Intel is just an over-priced, ego-centric company that has only OEMs to buy their chips, as well as a few marginal SOHO customers that build their own machines.
~Just my $0.02