The getPlainText() method, as its name suggests, only returns the plain text without any rich formatting. You should use the convertToRaw() and convertFromRaw() functions to serialize and deserialize the contents of the editor.
You can import them this way if necessary: (assuming you are using ES6)
import {convertFromRaw, convertToRaw} from 'draft-js';
If you need to export HTML instead, see https://medium.com/@rajaraodv/how-draft-js-represents-rich-text-data-eeabb5f25cf2#9260 (not sure you can import the contents back from HTML, though)