I wouldn't say how Cilan is being left behind, at least not to the point that you could get feeling how hes in background not doing anything. He still provides comedy and dynamic interaction in cast with his personality, use his pokemon like Pansage contributing to episode plots or when needing to battle villains etc.
Not so long ago he had tag battle with Ash against Soren and Ricko, helping them when trying to catch water type pokemon. And if i recall right he played some interesting role in Cheren debut episode too.
If you ask me its more that writers are now balancing things out after Cilan being much more prominent in first half of Unova sometimes overshadowing female part of group. By giving slightly more focus to Iris now since she is likely set up to leave cast after BW series making sure to give her sufficient amount of growth which could allow to partially close down her story and sent her away.
In comparison Cilan was barely developed as far as his connoisseur career goes, there were left unanswered quite a few things about procedure and way you can gain higher rank as trainer which identify compatibility between pokemon and their trainers to see if they're matched for each other.
He was left undefined as character on large part, not being build up for anything with future and direction in which he might go being highly precarious at this point.
Making it certain to some degree how writers might keep him pass 5th generation to add more balance and stability in main cast by giving viewers another familiar face beside Ash, in fulfilling role of older care taker, while being fleshed out more as character.
Than again more coherent and continuity based story as well stable cast was never one of writers main priorities, so things could go either way for 6th gen.
His goal is perfectly set up for him to travel around with Ash because his goal that he's working towards doesn't exactly require him to stay behind, meaning he can travel with Ash without feeling like his goal is being messed with opposed to how Misty and Brocks goals.
As far as Misty and Brock go not necessarily.
Perhaps in Brock case since studying to become doctor usually requires stay in one place. Although there exist traveling doctors and he could always go on adventure to learn about new ways of haling consulting with other docs but i digress.
But as far as Misty go since her goal is to become strongest water trainer in world being showed how her dream requires traveling and entering high level tournaments if you want to rank up as trainer with gym not being sufficient, there is no harm in going on venture to learn new things and advance like Wallace, Koga, or currently Cilan does.