The reason why the characters in Pride and Prejudice do not openly display passion may have everything to do with the author's purpose in writing the novella, and the formula that she chose to bring out specific themes.
Pride and Prejudice is not necessarily a love story, but a comedy of manners in the subtle formula of the Horatian satire. This means that the purpose of writing it is to expose the weaknesses in people that are often thought of way too highly,for example, the aristocrats and the upper classes.
This genre also aims to expose the qualities that may make the audience question themselves as to why they think so highly of the upper classes in the first place.
Therefore, Austen's focus was to satirize a stratus of society and not to involve the audience into a complex, sentimental tale.