Remember that Unova is supposed to be MUCH MUCH further away from all the rest of the regions. Remember, it was a plane ride instead of a mere boat ride. It could be that visiting Ash would take up too much time in their schedule. All four of them have their own business to take care of, too.
Thats not much of a problem though as long writers would create something which would be beneficial for their stories and plot of current series.Visiting Ash would be just additional benefit with primary purpose being to advance their dreams and story.
For example with Misty it would make sense to go to Unova for some water tournament or competition to advance her goal of becoming water pokemon master which cant be done at gym .Considering how Unova is isolated from older regions coming over there to catch new water pokemon which she otherwise couldnt also gives valid excuse to travel there too.
Incompetent sisters werent that much of problem when Misty used to travel with Ash and im sure they could take care of gym while she is absent or find someone else to take care of it.
For all we know judging by how long Misty wasnt showed she could have left gym already to go on a journey to further her dreams and learn new thing about water pokemon and battling gaining knowledge,Several gym leaders did that already.
With Brock considering how he wants to become pokemon doctor he could go to Unova with purpose of traveling to gain new knowledge about pokemon and techniques through which hje could help them preparing himself to pass necessary tests.Or he could hear or be invited by some famous pokemon doctor over there traveling to expand horizons on this area of expertise.
He could also decide to become traveling doctor instead of opening daycare center which would increase his chances of meeting up with Ash since he could travel through Unova for one of above mentioned reasons.
With May and Dawn admittedly its little harder to find some believable scenario to have them come to Unova now that contests are gone,but i guess on of them could come with purpose of trying to promote contests among people trying to have them become interested in new approach toward pokemon and battles along with update about their journeys being given and how much they improved.
In reality it doesnt matter how much some region is distant if writers create something which would justify their journey over there,with purpose of advancing your goals being one of valid reasons.
