Something will always come up that can be used as a rationalization by those who simply don't care to be bothered. That's why the date has been pushed in the past, and why Congress decided to set a deadline date, four years in advance, so that people have more than enough time, regardless of what happens.
There are stations that have already transitioned, as trials and as the vanguard of the transition, who will lose viewers to the stations that procrastinators will watch instead. Part of the planning involved the recognition that all stations must go pretty-much all-at-once (or at least within a small window), because of these competitive concerns. In addition, the entire city of Wilmington, NC, has already transitioned.
Stations have structured maintenance plans based on the transition. Our local Fox affiliate's analog transmission capability was destroyed a few months ago. In a normal situation, they'd have spent tens of thousands of dollars to replace it by now, but instead they patched something together and are operating a low-power for now. They need to know exactly when the transition is to happen so that they can make good decisions about what money to spend for what.
Industry has already bought and paid for the spectrum that the transition will free up. Companies have business plans to execute, capitalizing on the money that they've already spent, spent in consideration of a promise to make the spectrum available. The country must act in good faith. It cannot sell something, take the money, and then not deliver on the schedule outlined in the sales contract.
Most people have known about the transition for a long time, and some have even made expenditures in consideration of the date that the government told them -- expenditures that they perhaps would have preferred to make later. The government's action deliberately (and rightfully) prompted people to make some expenditures now -- it is unfair to reward people who refused to heed the government's warnings about the transition and punish those that did listen to the government.
The government worked with manufacturers on the transition, making commitments to the manufacturers that a market for what the government wanted the manufacturers to sell would exist. And it has until now, but if the transition is pushed out, then sales forecasts are totally scuttled. After representing to industry that the date was firm, and having industry incur expense to build inventories in consideration of that being the date, it is irresponsible to change the date.