Why did brock and dawn leave ash in the end of the last season before going in to black and white season???

i dunno i live in ireland

The Last Episode of Diamond and Pearl MIGHT be the last time Hikari(Dawn)and Takeshi (Brock) would be seen as Main Characters however they might make a reappearance in the Best Wishes Series!It Happens like that in Every Season Satoshi comes to the Region alone with Pikachu (after they said good bye to their friends in this case it was Hikari & Takeshi) then he meets new friends in later in the episode who are Dent and Iris who takes Takeshi and Hikari's place in the show.

dawns banery goes 2 wrld tour contests so dawn stays in sinnoh ash and brock went their sepret ways and brock became a poke doc. and ash took a boat with his mom and oak to the unova region.

dawn stayed because of the contests that were there and broke became a pokemon doctor so they both left ash but they get replaced in black and white

Like Misty & May, Dawn left at the end of DP, However, Brock does to, and neither of them come back as Main Characters. Their places are taken by Iris, a Dragon Trainer, and Cilan, a Gym Leader.

Dawn returns for 9 episodes in Best Wishes, kinda like May's and Misty's 5 episodes in the series following their departures. Brock, while not appearing in the Official Series, appears in the Cilan Special