Because it is the responsibility of the person trying to make the pass to read the situation and do so in a safe and fair manner. You don't get extra track rights just for being faster, and they are well within their rights to defend their position. Sounds to me like you're trying dumb moves and paying the price, as well as ruining someone else's day. Also, in many communities it is considered either rude or just very poor etiquette to flash your lights at people. If someone feels like you have a poor "attitude" then they'll probably defend even harder just to spite you. I know I would.
Flashing light is rude? Must be rude to drive like a professional than. Also, of course they have the right to defend, i’m not talking about defending, i’m talking about wrecking ppl on purpose just so you don’t pass, but just take themselves out of the race