Hard plastic was the latest, most modern material available in the early 1950's. Plastic WAS Fantastic!

 
My cousin opens some sort of plastic doll that Santa has left her

Until 1950, commercially manufactured dolls were made of composition (sort of a glorified paper mache) and porcelain. Hard plastic, which came onto the market as a development for World War II, allowed toy-makers to produce tiny, detailed, and very durable dolls, with new features such as weighted open-close eyes and walking mechanisms. These things had formerly been possible only for much larger dolls. 

The 8-inch size format was ideal for children. The dolls were lightweight and easily  manipulated by small hands, and yet were not too small for detail and ease of dressing. 

The small size meant small costs in materials, which allowed the dolls to be sold at a very affordable price. Nearly any child could afford an 8-inch doll, which sold for as little as 98 cents, and no more than $2.98. (I don't know exactly what this translates to in today's dollars, but I would guess it to be in the $8 to $25 range.)

About 1955, vinyl also came into use for doll-making. 

Each major doll-maker produced a model with vinyl head and rooted hair. In many cases, this was their most expensive model. For instance, Cosmopolitan's Ginger all-hard plastic doll cost $1.59, whereas the vinyl-head model cost $1.98.

However, rooted hair, once cut or damaged, was ruined (Wigs on hard plastic heads could be replaced), and the vinyl heads lacked the sensitive detailing of hard plastic. The early vinyl did not age well, either. Today the dolls with hard plastic heads are valued more highly than the vinyl-head ones, even though these are harder to find. An exception, of course, is the very rare mint-in-box, perfectly preserved vinyl head doll that occasionally comes onto the market.

(In fact, recently the WebDoctor warned that many old vinyl dolls are producing a toxic residue, and warns collectors to wash their hands after handling them. Click HERE to read)

About Competition

Most people describe Ginger, Vicki, Mindy, Pam, Lucy, etc., as "made to compete with Ginny." I am not sure this is a useful or correct way to look at it.

It is interesting that no one ever speaks of Madame Alexander's Wendy or Nancy Ann's Muffie as "made to compete with Ginny." Certainly they emulated Ginny, following the time-proven business maxim "To succeed, copy success!"

"Competing with Ginny" is based on a "scarcity" or "dog-eat-dog" model, in which there is a finite amount of money to be made. Each competitor grabs for him/herself, and whatever one gets, another loses. To some extent this was true of the Ginny-types, as it is true in any market atmosphere. In the dying phases of a fad this was especially true, but that did not happen to the 8-inch child dolls until the close of the decade. And then it was not so much that Ginny beat out Muffie who beat out Ginger. No, they all declined, due to the fact that the market had changed and there was no longer so much interest in ANY 8-inch child doll. The Barbie Era had arrived, as well as the Women's Movement, resulting in broader horizons for little girls, beyond just playing with dolls.

When an idea's time has come, it is popular enough to support a wide range of variations, models, and makers. This happened with the 8-inch dolls throughout the 1950's. In many ways each "competitor" actually enriches the "competition" through creating and furthering wider interest and demand. This factor is well-known in the world of automobile sales, where dealers prefer to locate on "Auto Row" then in an isolated location free of competition.

It was the same with children's toys in the 1950's. The presence of Cosmopolitan Ginger or Virga's Lucy on the store shelves did less to damage Ginny through competition than it did to expand her popularity through broadening the market.