The problem with the anime is that Ash is the protagonist. If they introduced one of the male characters as one of his allies, they'd butt heads. Making one of the game's guys, say Ethan, Lucas, Hilbert, or Nate, one of Ash's companions would take away from Ash being the main focus or too greatly diminish the value of the male introduced. For example, the episodes with Kellyn (Ranger 2) and Jimmy (Gold/Ethan) showed them both as determined males, but left them too similar to Ash.
The girls, however, greatly contrast Ash. Ash is essentially all the male characters, so his female companion is what needs to change each region. Dawn and May were able to focus on Contests, so they were much more pushed than Brendan and Lucas. Now if Brendan had been more like he was in the manga and entered Contests, I could see him traveling with Ash, but then you wind up with the awkward lack of an important girl.
Personally I thought that contests were one of the best things to happen to the anime. It gave the female characters a real place to shine and brought out their development much more than they had previously had. Unfortunately, because of the showmanship and fact that Ash's Coordinator companions are girls, Contests may seem sexist. I personally think that they did a decent job of avoiding this by making rivals males (Drew, Harley, Robert, Wallace, Kenny) and making Ash and Brock occasionally enter them, but at a glance it does seem more designated for females, which isn't fair because I'd love to be a Coordinator and I'm a guy.