It may be to try and reduce lounge hopping. Each lounge entry incurs a charge and obviously QR would rather see eligible pax enter just one lounge. Remember they are not generating any income on other carriers' Y pax through their own lounges.
For the few OW airlines flying to DOH, they do make a bit. But for BKK,, LHR, and CDG it is a choice of their own not to have any facilities for Y pax with status.
Is lounge hopping really that widespread? My impression is that most passengers will just go where the check in staff directs them to go.