Mostly history. Almost every single rider in cycling who has brought top performances has later been caught, admitted doping themselves or at least been credibly accused (for instance by team-mates) or their names wer found with doping doctors...I am new to this forum, so sorry for stupid questions. But why are you all so convinced that all the top riders / top teams are doping? Is this based on any verifiable and objective facts, or only your gut feeling?
So all the people saying it's unfair accusations: no. It's normal human's brain deductions. Also, doping tests have rarely brought athletes down. We know of extremely many cases where doped athletes have beaten the doping tests (they are often caught because someone talked, not because the tests caught them) and anti-doping agencies have explained why it is so hard for them to catch dopers: because they are usually behind in development, they aren't funded that well, and the limits of allowed substances are so high that people don't get accused falsely - with the downside that there is a good deal of leeway for most athletes in terms of microdosing at least.
The doping omertà, as incredible as it sounds, has been proven to work.
Personally I am careful with "all of them dope". I don't know that. My personal believe is that there are huge differences in what the riders are doing and how far they are willing to go. I have no idea about percentages. I expect some to be more or less clean. But I don't know how many.