Its the same reason Brock's breeder goal is hardly focused on, he's not here to be a main focus, just to be Ash's friend and get roles in fillers, which is similar to Misty.
Even so, the show still could've focused on their goals more with Ash's still getting much more screentime.
Well I do think Misty will appear in Sinnoh at some point, so you'll have to just wait and see. Sadly, I do think it'll just be because of Togekiss, which is lame because there hasn't been a major Misty story outside of the Togepi line for a good awhile.
I honestly hope Misty isn't invovled with Togekiss, especially if it's not hers. That'd just be too convenient. (Well, if the writers pulled it off well...)
But you just can't expect people to forget the original series. A considerable portion of the fanbase grew up with the first 276 eps., the first three leagues, the first 251 Pokemon, and the first traveling group, and while some have moved on, a large number of old-school fans still wait to see the original trio in an episode together again.
Many people stopped watching before the end of Johto, some I can imagine stopping after Kanto.
I wouldn't mind if the original trio got together with May, Dawn, Tracey, and Max to accomplish some kind of mission. (The entire group could split up, with Ash, Misty, and Brock being in one group.) Not that you were suggesting this, but I wouldn't want a whole region where they're traveling together. (That'd be like one step forward and two steps back to me...)
On the same token, unless May has "her plotlines tied up" by the time of her next cameo, it's assured that her fans would want to see her again to see her plot finish up as well. Poor Ash...while the females accomplish their dreams, he is stuck in a perpetual cycle of riinse and repeat for each league until the end.
I don't see how May, Misty, and Dawn's dreams are anymore accomplished than Ash's. And Ash doesn't mind the journey to becoming a master at all, so...