Sounds like a pretty good reason to me. Hard to broadcast both on one channel.
From what I've seen , most stations (VHF at least) are up and running digital on their new frequencies, which they will stay on, and quit broadcasting on their old frequency. Then those VHF frequencies are going to be used for emergency services like police and fire, and also commercial services like cell phones. The first reason was one of the main reasons given for the change, but I have a feeling that the bandwidth they're selling commercially was more likely a better reason as far as the feds were concerned.
Sure, money is a big piece of this.. But just take a look at the 800 MHz cellular vs Public Safety fiasco for a minute. Otherwise referred to as "Sprint/Nextel REBANDING" in my current line of work.
Nextel gobbled up every available 800 Mhz frequency that they could get their hands on back in the early days of iDen technology. And over the years Nextel frequently caused interference to their Public Safety neighbors on adjacent channels. So the government basically ordered Sprint-Nextel to FIX the problem. In exchange for paying for this nation-wide mess, and I do mean MESS, Sprint got a nice chunk of 1.9 GHz spectrum all to themselves.
What I'm getting at is this.. The government does not want a repeat of 800 MHz Rebanding. EVER. It is already way behind schedule, with multiple cases hung up in mediation and other litigation.
With the advent of DTV, and the oppurtunity to fit 5 channels in the same spectrum where 1 channel used to fit, this basically opened the door to free up a huge chunk of 700MHz spectrum to set aside first and foremost for Public Safety.
Understand that the DTV mandate was not necessarily because of 800 MHz reband. The two processes have actually been running concurrently for several years. It's just that the DTV mandate ensures that the 800 MHz fiasco won't happen again, in my opinion anyway.