Because the stainless steel construction of the DeLorean makes the Flux dispersal big enough to surround the car. It also needs the plutonium to generate enough energy to make everything inside the Flux dispersal (think of it as a time bubble) travel through time. Previously, he had sent clocks forward in time to test the Flux Capacitor (which is why there’s hundreds of slow clocks in his lab), but he didn’t do it in the DeLorean. Doc even says he built the time machine INTO a car, not turned a car into a time machine. My theory is that he had the Flux Capacitor hooked up to a bar fridge. Zemeckis and Gale have admitted that Doc’s original time machine was a fridge, so there’s a little bit of evidence that this could be true. If he is only sending clocks and the Flux dispersal only has to envelope a fridge, he wouldn’t need as much power. He’d need a good power supply, but not as much as plutonium. I think this was the actual purpose of the power amplifier Marty blows up in Docs lab at the beginning of the movie. I’ve thought about this a lot. Lol