QUOTE (alexbiker @ Mar 14 2010, 11:46)

Lots of reasons: Same cars but with some truly terrible setups.
Push to pass button.
Definitely helps, but they had plenty of passing before push-to-pass
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Hard tyres, iron brakes, less downforce - longer braking distances so more time to make a pass.
Why do you think the tires are any harder than F1 tires? Heck, when F1 had grooved tires, I'd bet money that the F1 compound was harder than the Indycar tires to keep the grooved treads from squirming and disintegrating.
Why do you think they have less downforce? Traditionally, with the gigantic venturi tunnels, Indycars were capable of much more downforce with less drag.
As for iron brakes -- their effect on braking effectiveness is relatively moot.
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Less downforce - less turbulence as the air isn't worked anywhere near as hard.
Less downforce - less pace lost when following.
Again, do you have any evidence that there is less downforce? Or is your reasoning a result of "F1 is the best, therefore it must have bigger tires, stickier tires, more horsepower, more downforce, etc. Why? Because it's the best."?
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Ground effect venturis - not in any way the same as a diffuser, much less susceptible to wake turbulence.
Ding, ding, ding! We have a winner! Yes, one the reasons it's much easier to pass in Indycars is the tunnels.
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No DDD - they were supposed to banned in F1 (a loophole was left, which made them legal) because they cause so much turbulence.
Hahahahaha!!! Do you know why F1 cars have diffusers? Because the F1 regulations do not allow them to have ground effects tunnels, ala Indycars. The diffuser is a band-aid! If the designers had a choice between a diffuser and Indycar ground effects tunnels, they'd choose the tunnels in a heartbeat. Once again, this thinking seems to come from the logic that "F1 = best. Therefore, if F1 has it (e.g. DDD), it must be the best". No, a lot of F1 engineering choices are because they are the best solutions given F1 rules, not because they are the best solutions overall.
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Worse drivers.
weak answer
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Most of all: defensive lines are not allowed at all.
This is another important point. You could also call it enforced good sportsmanship.