Reason: the anime promotes the games. More specially, because the roles of Cilan and Iris in the games.
In BW, Cilan is the main Gym leader of the Striation Trio in Black and White. And Oshawott is the most popular starter what caused that Cilan is getting popularity. However, Iris is only the 8th Gym leader and only in Pokemon White (Black is more popular) what caused that Iris isn't that popular. In it, Cilan gets more screentime than Iris.
In B2W2, the situation is very different. Cilan isn't a Gym Leader anymore while Iris is Champion* in the both games. Although Iris can't be CHampion in the anime because that means that Iris must age (impossible now) this caused that Iris were more popular than Cilan.
Notice that when a character doesn't really promoted the games the characters gets less screentime than usual like with Brock in AG and DP.
Ash is officially Red** in anime version, and unofficially Brendan, Lucas and Hilbert***, meaning that always is promoting the games and it is always popular.
*This is why Iris in the anime got a 600 BST pokemon It's Dragonite because that thing it's popular.
**Ash acts like a Red who travel for three regions, Kanto, Orange Islands and Johto. To Ash start adapting other game characters, Ash and Piakchu are "resetted".
***Because the writers acan't reallistic promotes the sequels in the anime Ash is acting like Hilbert during all the generation.
@Haunter, @Taidow, Cilan's lack of screentime don't help him staying for Gen VI at all.