Because of Walt's vision to never serve alcohol in the parks. Honestly, as much as I appreciate keeping the integrity of Disneyland... this is a great move.
Considering alcohol has been served a plenty at every other Disney park (including right across the esplanade from DL), and there's really been no issues from it, not serving it in DL has always felt like a non-existent threat. Not to mention Club 33 has been doing it for decades, so the park has never been truly dry. Add in Oga's Cantina and yeah, there's places in DL which I wouldn't mind having a drink, and honestly Blue Bayou is one of them!