From what can be told, this is simply due to the proliferation of Haribo's Gummibears. Originally invented in German in 1922, they were called Tanzbären (Dance Bears) and represented the first sweet gummi candy or jelly candy introduced to the world-wide market.
The actual process they are based on is older than that. The recipe for a gelatine based sugar candy was first pioneered by Charles Gordon Maynard in 1909 in Great Britain but Maynard used it only for the production of wine gums, which differ from gummi bears by having additional tastes of sour added and containing actual wine (the alcohol evaporates when cooking it). Maynard wanted specifically to market his gums as a wine product for he was a staunch teetotaler and therefore wanted to break into the market for wine consumption but without his consumers actually consuming alcohol.
Enter Johann "Hans" Riegel Sr., confectioner from Bonn (Haribo means Hans Riegel, Bonn). Riegel was a trained confectioner and bonbon maker. After having served in the first world war, he became convinced the future of confectioner and bonbons was in highly industrialized and standardized manufacturing techniques – a trait, which in terms of broader social and cultural history of Germany post-WWI was quite common among entrepreneurs. Riegel took Maynard's receipe and left out the wine and instead added more sugar and fruit flavor. Additionally, he gave the candy the shape of a bear in a noticeable effort in marketing it to children.
At the time, marketing as we know it in the modern sense was something that as a discipline and art was only just emerging. With notable influence from the US where marketing through the real emergence of a consumer industry as well as through the lessons of propaganda in WWI had effectively been revolutionized, Germany set out for its own "marketing revolution", the most famous outcome of most people will be familiar with: the Nivea Creme.
Nivea is brand. Everybody knows the blue little can and you can recognize it just by seeing the blue. This idea of establishing a "brand", simple, recognizable and iconic. The idea behind that that also influenced heavily why gummi bears are bears, is the idea that unlike wine gummies which were praised as well-tasting and marketed to wine drinkers, you need something that has at first nothing to do with the product itself but still pulls in consumers into recognizing your brand as uniquely your brand.
This, as mentioned, has to do with the lessons learned by the various war-waging powers in WWI in terms of their propaganda effort. Propaganda, as a sort of state sponsored advertising, though people that emotionally associated short hands in terms of recurring images, characters, and forms can immensely help in selling the intended message. This was something that entered popular and especially economic culture thereafter. In terms of arichtecture f.ex. this found entrance in what is today called Neue Sachlichkeit (New Objectivity) where the decidedly pragmatic and function-oriented style of building became a sort of selling point that entered culture as a generally "futuristic" oriented aesthetic that would become heavily associated with the new style in the rejection of the fin-de-siecle aesthetics.
But back to Hans Riegel: Riegel under the influence of all these cultural forces realized that in order to market his gummy candies, he needed a group to market it to and he decided – because of the sweetness etc. – to go for children. That is why he shaped them in bear form, gave them different colors, and called them dancing bears.
The gummi bears were an immense success in Germany and with the end of WWII started a world-wide success story. Since the 1960s they are known as gold bears by Haribo but even before there had been copycats using the bear shape as marketing technique to get children to buy and eat them. Hence the iconic image might not be the absolute standards (there's also jelly beans in the US context where gummi bears took off in the 80s with both yuppies and children that watched Disney's gummi bears) but it certainly became an iconic shape associated with gummi candy.
Sources:
Tim Richardson: Sweets: A History of Candy.
Catherine C. Fraser, Dierk O. Hoffmann: Pop Culture Germany!: Media, Arts, and Lifestyle
Could you provide some sort of reference to the bear being standard? Different shaped gummy candies are also popular, such as Swedish Fish, gummy worms, and gummy lifesavers.
