Ya know why? Because it's not confidence. It's desperation. Brown built that time machine out of stolen uranium that he bought from Libyan terrorists. It doesn't matter how great his invention is -- the minute he goes public with it, people are going to ask how it works. Then they're going to ask him where he got the uranium, and then he's going to prison. Brown isn't working for money or recognition, he's working for his own satisfaction, his own identity. Let me paint this picture for you.
Every night since Nov. 5, 1955, when Doc Brown's failed suicide attempt (sure, buddy, you were "hanging a clock"), Brown has dreamed of mattering. Dreamed of creating something important. But every invention, be it a dog-food opener or even a simple amplifier, ends in utter disaster. Night after night he stares blankly at the ceiling until the cracks begin to writhe. Someday, he whispers to himself. Someday.
Then, finally, a three-decade-old vision crystallizes. It's never been clearer: nuclear power. Chrono-displacement. A stainless steel body. Giant heat vents for some reason. The vision burns inside him so hot he can barely breathe, and in his madness, he reaches out to terrorists to attain the supplies required. By the time his passion cools, it's too late. His path is set. The time machine is built, and whether it fails or succeeds, he's doomed, because a man like Doc Brown can't be locked up. No. He'd rather die.
The results of a 40 MPH Crash Test.
Hence the DeLorean.
So he builds the time machine. He puts his dog in it. He invites the weird teenager who's always at his place to come over and film it, and he forces that innocent child into his suicide pact. Worst-case scenario, he'll bring a slave with him to the afterlife.
But luckily it does work and Marty gets to go back to the '50s and almost fuck his stupidly hot mom.