which is apparently due to film in April of 2013, is putting a crimp in the budget for series 8, hence the stringing waaaay out of series 7.
I dug this up on Bleeding Cool...
In theory, all thirteen episodes could run right the way up to Christmas in an unbroken streak but it at least feels more likely that this year’s split strategy will come into play again. I’m expecting we’ll see six or seven episodes in the Autumn, with the remaining seven or six held back until the start of 2013.
http://www.bleedingcool.com/2011/09/22/doctor-who-delayed-next-years-series-wont-start-until-the-autumn/
-and this was back in 2011.
I don't think Moffat can be held accountable for the delay, he doesn't seem to have had any trouble getting any of his previous series out on time. I think this can be laid at the BBC's doorstep, they are the ones who decide the schedule after all.