Alcohol is thought to cause injury to the mitochondria which contains AST but not ALT. In addition, in chronic alcoholics, pyridoxine (vitamin B6) deficiency may reduce the synthesis of ALT more than AST because the former is more B6-dependent (1).  

AST/ALT ratio >1 may be more common in advanced alcohol liver disease (e.g. cirrhosis) than in the setting of high alcohol consumption without severe liver disease (2). 

Also, remember that AST levels greater than 500 U/L and ALT levels greater than 300 U/L are uncommon in alcohol-related liver injury.  In this setting, other causes such as acetaminophen toxicity should be excluded (1).

References

1. Johnston DE. Special considerations in interpreting liver function tests. Am Fam Physician1999;59:2223-30.  https://www.ncbi.nlm.nih.gov/pubmed/10221307  

2. Nyblom H, Berggren U, Balldin J, et al. High AST/ALT ratio may indicate advanced alcoholic liver disease rather than heavy drinking. Alcohol &Alcoholism 2004;39:336-39. https://www.ncbi.nlm.nih.gov/pubmed/15208167