... and cost vs usefulness, especially with cameras offering higher and higher usable ISOs with each new generation.
Most of my lens kit is primes. Three have I.S. The only one that doesn't have I.S, that I occasionally wish did is the 135/2. The shorter lenses I really don't need it. Especially if it were to add size, weight, complexity and $400 to $600 to the price of the lens.
I think Canon might add it to the 135mm some day, and to the 200/2.8 and 400/5.6. The 180mm Macro will probably see it added sometime, too... But I'm not really sure if I'm sold on I.S. for macro yet or not. I'm still using a non-I.S. 100mm Macro and don't have any plans to change (it's on a tripod or monopod mostly, when shooting at higher magnifications). I just don't see the 85mm, 50mm, 35mm, etc. needing it at all.