With BAC, I can look at a chart and know I'm not legally allowed to drive after more than two drinks. I might think I'm fine to drive, but I know I'll get a DUI if I get pulled over, so I don't try.
Without a clear objectively defined and easy to asses limit, people are left to decide on their own if they're impaired before driving. Drunks are notoriously bad at accurately assessing their abilities. The BAC system prevents all the (law abiding) people who think they're fine from driving when over the limit because they're afraid they'll get a DUI if they happen to be pulled over. Some of these people are actually impaired and have effectively been kept from driving under the influence. A reactionary test would send them all on their way, and they'd never find out if they're actually impaired until they get pulled over or crash.