The trouble is obviously that Steven Moffat is incompetent. Russell "hooray for me!" Davies would have had fifty double-length episodes written and sent off in the same amount of time that it seems to take Moffat to write a single Christmas special.
Seriously, there have been rumblings about this for a while. Moffat has only managed to complete one full season in all the time he's been show-runner (Series Five, his first), and only then because he had the lead-in from Davies doing the Tennant specials. Ever since then Moffat has become notorious for missing his deadlines. The split of Series 6 into two halves was allegedly because he'd spread himself too thinly across both Doctor Who and Sherlock, but it seems like he just can't get it together now.
To be honest, I think they'd be better off simply admitting that Doctor Who only has six episodes each series from now on. None of this 'Series "x" Part "x"!' business.