I had hepatitis a over 12 years ago as an teenager, why are you not allowed to donate blood, when the virus is completyly out of your blood in a year?

Hepatitis and blood. The red cross disqualifies anyone who has had any hepatitis (a, b, c, d or e) or jaundice after the age of 11. Exceptions include non-viral causes of hepatitis or jaundice such as gilbert's disease. Presumably the reason is to cast a wide net over all cases of viral hepatitis to ensure not missing a case of non-a hepatitis.
Likely other reason. At this point in time, to my knowledge, hep a infection that has resolved is not an indication not to donate blood. The immune system is effective and there is vaccination. However, you may have had another infection/issue, like hepatitis c which has no vaccine, and can be transmitted. Also, if you went to europe, you might have been exposed to "mad cow" disease, and cannot donate again.
True. Are you sure that is the reason?