I bet they weren't too unhappy about not paying an agent/referral fee to these "price comparison sites" (that are really often travel agents themselves, or are at least getting a commission from those or the airline directly).

For related reasons, Easyjet or Ryanair (I forgot which one) was not available for booking through the regular GDSes for the longest time, which made them unavailable for booking through traditional travel agents not directly integrating with their proprietary inventory system. I bet that was as much about not wanting to pay the incumbents' fees as much as it was about being "modern API forward".