Why is heroin not used more for OST compared to methadone/buprenorphine?

I live in Ireland and work as a GP trainee. There are two doctors in my clinic who are qualified to prescribe methadone. From talking to them, they say that while methadone is very effective for many patients, it's also a hit and miss of others. Methadone relieves the withdrawal symptoms but not the psychological cravings and so many still end up taking heroin on top and/or other substances.

Having studied drug addiction for a project in medical school, I did learn that there are several countries in Europe (the Netherlands, Switzerland, Germany) that offer diamorphine for those who don't respond well to methadone or buprenorphine.