Please bear with me as I've never read any of the LOTR books or the Hobbit, only seen the movies so far...
But when the sunlight fades and the dwarves think they've lost their only chance to enter Erebor using the secret passage and the key, they turn around to head home. Why is it that the dwarves would be unable to enter Erebor the way they did years ago, using regular doors to get inside? I'm assuming that's how they entered Erebor years ago?
The Mountain did indeed have a main entrance, which the Dwarves called "the Front Gate". However, the point of the Dwarves bringing Bilbo was that, as far as they knew, the dragon was still alive and active. This could have posed a problem for their entrance:
... We none of us liked the idea of the Front Gate. The river runs right out of it through the great cliff at the South of the Mountain, and out of it comes the dragon too—far too often, unless he has changed.
(The Hobbit, Chapter 1, "An Unexpected Party")
When Thorin and Company arrive at the Mountain, a few of them (including Bilbo) go to scope out the gate:
Thorin sent out a scouting expedition to spy out the land to the South where the Front Gate stood. For this purpose he chose Balin and Fili and Kili, and with them went Bilbo. They marched under the grey and silent cliffs to the feet of Ravenhill. There the river, after winding a wide loop over the valley of Dale, turned from the Mountain on its road to the Lake, flowing swift and noisily. ...
They did not dare to follow the river much further towards the Gate; but they went on beyond the end of the southern spur, until lying hidden behind a rock they could look out and see the dark cavernous opening in a great cliff-wall between the arms of the Mountain. Out of it the waters of the Running River sprang; and out of it too there came a steam and a dark smoke. ...
"The dragon is still alive and in the halls under the Mountain then—or I imagine so from the smoke," said the hobbit.
"That does not prove it," said Balin, "though I don't doubt you are right. But he might be gone away some time, or he might be lying out on the mountain-side keeping watch, and still I expect smokes and steams would come out of the gates: all the halls within must be filled with his foul reek."
(The Hobbit, Chapter 11, "On The Doorstep")
As the Dwarves get more and more desperate (in the book, not the movie, they spent at the very least several days and perhaps a week or more trying to figure out how to open the secret door), Dwalin suggests sending Bilbo through the front Gate: a prospect that dreadfully upsets Bilbo:
"What is our burglar doing for us? Since he has got an invisible ring, and ought to be a specially excellent performer now, I am beginning to think he might go through the Front Gate and spy things out a bit!"
Bilbo heard this—the dwarves were on the rocks just above the enclosure where he was sitting—and "Good Gracious!" he thought, "so that is what they are beginning to think, is it? It is always poor me that has to get them out of their difficulties, at least since the wizard left. Whatever am I going to do? I might have known that something dreadful would happen to me in the end. I don't think I could bear to see the unhappy valley of Dale again, and as for that steaming gate! ! !"
(The Hobbit, Chapter 11, "On The Doorstep")
Instead, of course, the plan is for Bilbo to break in:
which he does successfully (twice, in the book, before the dragon dies).
Because there was a huge dragon sleeping in there, and he would kill them.
A dragon could have easily smelled and killed them instantly. They needed to find a different, secret way to get it and get their business done without having the dragon wake up.
The only way to do that was to go through the secret passage you saw on the second Hobbit movie.
