It didn't started like that:
Roy Lichtenstein. Look Mickey
Andy, a comic artist, who has great works in his site, such as this Abstract Comics He gives a time line of comic in history from antiquity (300-1000) till the early years of the 20th century...now, for sure his got his own interpretation of what comics was in early centuries...worth watching if only for the beauty of some ancient items...
The 18th century term for speechballoons was 'labels'. in his site you can study various experiments and variations and the gradual change from bands to balloons. Even the later examples sometimes have tiny leftovers of the 'bands' on the other side of the balloon.
During the 18th century, British caricaturists changed the shape of speechballoons from gothic speech-bands or flags into fluffy balloons, our modern speechballoons. The gothic form of speechballoons are speechbands, flags, scrolls or sheets of paper, the modern form of speechballoons are balloons, but also little rectangles, often rounded at the edges, or simply little blocks of text above the heads of the speaker etc. via coudal
1404 - Konrad von Soest - Altar in Bad Wildungen
more readings: History and Theory of the Graphic Novel