Because price-to-performance is much better on an HD3870 than the 8800 Ultra.
The Ultra may be the best, but it costs $700. The HD3870 costs nearly a third of that. You do the math. If an HD3870 does roughly 65-70% the power of an Ultra, for a price of around $230, that means to fill up the other 35-30% in an 8800 Ultra would cost an extra $470. Is that worth it to you?
That's what's called "diminishing returns", it costs more and more to go up in performance more slightly. At some point, you gotta step back and think "wow, this card is a ripoff, I should get xxxx card instead because it offers 90% of the performance for nearly 2/3 to 1/2 the price!"