It's perceived as a failure because it was a failure. If anything, the US is still too restrictive when it comes to alcohol policy. I hope we see the drinking age lowered back to 18, and remove most geographic and time restrictions on the purchase of alcohol. (i.e. Dry counties, purchasing on Sundays) It's funny, I don't see too many people championing the war on drugs as a public policy success.
A higher drinking age has been shown to lower drunk driving fatalities. https://www.webmd.com/parenting/news/20080702/age-21-drinkin...
For instance liver disease deaths were decreasing until 2008 [1] Would/was lowering the drinking age have an impact ? I don’t know, but I just think there are wide and long term effects on these decisions, and judging by a single metric would not be fair.
[1] https://www.washingtonpost.com/news/to-your-health/wp/2018/0...