Is because they are filled with human waste. This would explain why the 24th escalator was finally fixed, only to be shut down the next day.
For more gnarly details: http://m.sfgate.com/sfchron/db_41686/contentdetail.htm?contentguid=NOzvZGvn
Now we no longer have to wonder, but at what cost this knowledge?
Microhood SF 
So gross. But I’m glad to know what’s been the cause of those persistent outages.
Correction! Thanks to Mission Local, we now know the true reason the escalator is so unreliable at 24th: http://missionlocal.org/2012/07/bart-answers-whats-up-with-the-broken-escalator/That said, I still think it’s waste-related…