I read that there was concern about the pressure that would be placed on the rest of the system if the water was abruptly shut off - but I wouldn't be surprised if locating an appropriate shut-off valve was an issue, too.
During the drought of 2009-11(ish), the city mandated that sprinklers only be used on Mondays and Thursdays, which helped trigger an increase in water main breaks around the city. A report found that the swings in pressure on the city's aging pipes made them more susceptible to failure.
http://latimesblogs.latimes.com/lano...ort-finds.html