Why won't Trump run? In his written remarks, Trump said that business is his passion, and that his decision was made despite strong poll numbers. Actually, Trump's support has been in a state of collapse. Trump declined to run for the reason almost everyone else does: He does not believe he can win the nomination.
Elephant Watcher was a "Trump skeptic" from the beginning, never giving Trump more than a 1% chance of victory. His vulnerabilities were clear from the start, regardless of national primary polls. It was bad enough that he was not viewed as an electable candidate; his identity as a Republican was also in doubt.
Consequently, Trump's departure alters the field only slightly. Christie has taken Trump's 1%, bringing Christie up to a 63% chance of winning the nomination. Christie will step in and take the role from Trump of the brash, "tell it like it is" candidate.
Was Trump's candidacy just a publicity stunt? Probably not. Trump did gin up a lot of publicity and controversy, but he also did a lot of behind-the-scenes work that would not have made sense if he was not seriously considering a run. Most likely, Trump tested the waters at first, was thrilled by his high poll numbers (for awhile he led in the national primary polls), and set to work on a run for the presidency. Then, as it became more clear that Republicans were not buying his candidacy, he decided to save himself the humiliation (and work) of a failed presidential run.