it's supposed to be quicker, so the BBC can show it. Sky aren't bothered if a match overruns on its cricket channel (and almost all do), but the BBC need to have them wrapped up on time. I believe teams have runs deducted if they fall behind the over rate?
The cricket board made a terrible mistake selling the game to Sky just after the 2005 Ashes. One day half the country is watching, next the audience has dropped off by I think about 80%. Sky's money has its advantages no doubt, but putting it on channels where few kids get to see it, they've lost half a generation of future fans. They're desperate to undo the error.