] I suppose it would seem more "manly" for modern audience to have Bingley and Darcy on horseback !! ;-)

And Bingley asking Darcy his opinion about Netherfield establishes a pattern in their relationship

From a visual/film point of view, having the men on horseback also allows us to see more of them, rather that just their heads or upper bodies if they were together in a carriage. And another thing they did with this scene (and the other riding scenes) is to try to demonstrate that Darcy is the wealthier of the two men -- his horse is evidently the better horse and Darcy the better rider.